How to deal with 2v2 rushes?

I play on a 2v2 team as Protoss, my ally is Terran.

1v1 rush handling is fairly straight-forward, and if you're prepared for it the defender gets the advantage because of quick reinforcements, high ground, etc.

In 2v2 however, when a rush comes, it's twice as strong, and the defender is at a disadvantage because:

• It's usually too early to join armies with your ally, so it's 2v1.
• As Protoss, when an early double rush comes in at 6 minutes, they are right at my production facilities. It's easy for one of them to deal with my units, and one of them to take out my pylons very quickly.
• There are more options for the opponent to see high ground: hallucinated colossus, overloards, or because the army is so big, just running straight up the ramp.

For example I just lost to a 6-minute marine+roach attack on Monlyth Ridge. We scouted and knew it was coming, but I don't know what we could have done to stop it. They killed my pylons while I tried to hold them off, thus I couldn't reinforce, thus they didn't even have to kill my base before going to kill my ally.

The typical answer to holding on in a 2v2 game when one ally is dying is: Die as slowly as possible. This is excellent later in the game, but against decent opponents at about 6-8 minutes, this isn't very powerful / possible because:

2. I have very few units to distract them with.
3. My map coverage & economy is not enough to start building dark templar or other surprise tech.
4. This early, they have a very clear idea of what options I have, so they are not afraid to just march into my base.
5. Luring them away from the base is too risky; without any defense in my base they will run in and kill production / economy.
6. Once they kill my production (which doesn't take long), they don't need to bother killing my buildings. They can march straight to the ally's base. They only need to keep a couple units in my base to prevent me from advancing.

My 2v2 team tends to win as long as we can hold off 6-8 minute rushes. Here are some ideas but I would like to hear your comments before I throw away lots of games experimenting:

• More sentries for forcefield. The advantage is this doesn't cost much of a compromise to my build as I can build them just as I build any other gateway unit. However, it only delays the inevitable and doesn't allow me to ramp up my production. It also prevents me from helping my ally if they go to him. So I am pretty scared to use FF in this scenario.
• Hallucination. They won't have detection at this point, and if they arrive at my front door and see 2 immortals and 6 stalkers, they may just run away. If not, it could absorb enough of their fire to buy us more time than a force field.
• Turtle. My ally and I prefer longer games so we just want to be able to hold off rushes. So should we do it decisively by turtling hard? Especially on Monlyth Ridge, could it be that doing a forge-first opening and quick-ish expand is the best way to fend off their early attacks? The idea would be to sacrifice early units, put all resources into defense and benefit from a healthier economy and play the game on our terms. But I am not sure static defense is strong enough for rushes like this anyway, and it's a lot of investment.

So, how does a 2v2 team defend against rushes like this as the different race options?

[Update, after accepting Sorean's answer]:

I want to say how I integrated Sorean's answer into my thinking and on the ladder. I think you can boil the issues down to joining armies at your base means giving up defensive advantages we're used to in 1v1. These advantages boil down mostly to positioning / base geography. Without that issue, for example on Discord IV, the defense strategy is the same as 1v1.

So the response is that, if you want to compensate for the disadvantage, then you'll need to emphasize more heavily the defensive capabilities of your race in a 2v2 defense. Fortunately Blizzard has given enough options to make up for the disadvantage in 2v2 (I think...). Note that I really don't know much about Zerg or Terran, but I'll try to list this:

1. For Protoss:
   1. Sentries with force field - if you can't completely block the opponent, you can at least choke them into a more favorable position.
   2. Cannons, though usual cautions apply for these costly defenses. But in some defensive positions cannons can be favored over units for the amount of damage they deal, and they are useful later in the game to detect burrowed units / dark templar.
2. For Zerg:
   1. Static defense (but usual cautions still apply of course). Zerg can benefit even better because of being able to move static defense.
   2. Creep
   3. Queens with transfuse. Think spanishiwa and how he manages to defend with almost no units against a 4-gate.
3. For Terran:
   1. Bunkers
   2. Siege tanks. Because siege tanks can arrive earlier in the game than anything else with such a long range, they are great for defending main / natural bases.
   3. Supply depots. Not sure if this really deserves to go on this list, but the ability to raise / lower a supply depot can be so useful in making building positioning work in your favor, and out of the opponents' favor. We are starting to experiment, putting the 2nd supply depot in the protoss base if we're walling in.

2v2 games are a unique beast in the sense that you need to change your style of play. My 2s partner and I used to die a lot when it came to these early quick game rushes, and on the flip side, have rushed others as well to vary up the play style.

You mentioned that you were able to scout that they are doing a rush, you could mix in your ideas of how to survive to get a good counter to a rush. (Going to be using Monlyth Ridge as the example map and your match up as reference PTvTZ)

At 6 minutes (I assume in game time) they can probably have 10-15 marines and 5-9 roaches.

• If you are confident in your micro, forgo static defenses and simply try to out produce your opponents. At 6 minutes, you can get your defensive 4 gate up. Although it's really close. A mix of stalkers to shoot from the high ground and sentries to hold off the ramp should be able to hold them off. Even a few zealots mixed in there to tank damage while sentries pick away could suffice as well. (So you'd have ~2-3 sentries, 4 stalkers, 1 zealot)
• You shouldn't be afraid to get sentries and use force field. As a protoss player you want to have a fair number of sentries mixed into your army anyway later in the game. The force field only lasts 8 seconds so it is not a huge amount of time to stop you from going to help your ally. If used correctly you may even be able to trap a few of their units in your base (although against a roach/marine army that would just give them high ground vision so not really helping you there).
• You mentioned that your ally is terran, there is very little cost to investing in an early bunker or even two (even with the recent patch, he's only going to lose 25 minerals in the long run if he salvages them). So long as your ally is quick on the repair then he should be able to hold off the rush long enough for you to build up an army and flank with him exiting his bunkers to assist when you believe the two of you have enough to take out the attacking army. (Ally if matching opponent would have 10-15 marines as well maybe a little less because of the bunker)
• I'm not sure how your buildings were laid out, but pylons usually shouldn't be in attackable range, although the new patch doesn't help this point much (which I personally think is a pretty stupid nerf to the race, but I digress). Try to ensure that you have overlap on your production buildings.

Assuming it played out with the above, if the enemy ran to your base first, you force field him off, pick off a few units, or even weaken them as the two of you build up an army to crush the attack since your reinforcements will arrive much sooner than the attacking force. If they attack your ally first they run into a face full of bunkers that are getting repaired and your defensive 4gate should produce enough units to kill that army that they are sending, that or enough to force a retreat.

Going a forge first fast expand with cannons in this case you'd probably need 4 or 5 to just hold off that army if your ally responds quickly enough with reinforcements. Also in this case you are forcing your opponents to either deal with your cannons (with a sentry on the high ground to your ramp to prevent a run by, but if your wall is good enough on the low ground a run by would sustain massive losses) or to attack your ally (who again should have 2 bunkers and since you were attacked first, have ample time to get SCVs down to repair. If you go with the forge first opening I would suggest getting at least 1 sentry and a 2:1 stalker:zealot ratio for the first little while as you'd try to stay close to your cannons when trying to engage.

The answer is actually a lot simpler than all of this.

Stop trying to play as two players. Consolidate your armies. They are doing it, why can't you? To re-iterate: The most important thing is that early on, you should be working as close as you can together to prevent the situation you are describing. Scouting 2v2 (you said you did this) is probably twice as important as in 1v1 (and it's already very important).

In fact, one of the most powerful counters to this is to go on the offensive as one and separate their army.

You have a problem in your assumptions.

You are not supposed to stop the rush. You can't it's 2v1.

You are supposed to delay the opponents as much as possible. Make them busy while your ally builds up his economy/force. He should be then able to easily crush at least one of the opponents and the other one should be behind on economy (or at most equal).

Plus, if you save at least some probes, then you can start slowly rebuilding.