As a 2-handed weapon wielding tank, is it more effective to specialize blacksmithing or enchanting?

I'm trying to play a proper tank character, but perks are indeed limited.

The plan is to go 2H, thus specializing in Two Handed indeed, Heavy Armor, a bit of Block and then I'm undecided between Blacksmithing and/or Enchanting.

Would I be more effective with enchantments on instead of fabricating/refining my own gear? What do you suggest?

Both are really important, honestly. You will get perks quicker than you suspect, especially because those 2 skills are really easy to power-level if you have disposable income. Fortunately, a skilled enchanter and blacksmith will be able to buy materials and sell the finished goods to the same merchants for profit.

Early on, invest most of your perks on the stuff that keeps you alive (via boosting your survivability or your ability to kill quickly). After 10 or so levels of nothing but weapon, armor, and blocking perks, start smithing and enchanting. Make iron gear (daggers are most efficient) and enchant them with whatever you know that has the largest impact on their value. Sell those daggers to get a nice profit. Work you way up the right side of the smithing perk tree (because that's the side specifically for heavy armors).

The reason smithing will be so important (beyond the synergy with leveling enchanting) is that improving weapons/armor after you get their respective perk is a huge benefit. With my smithing in the 80s, I am easily adding 20+ armor to each piece of armor I wear, and 10+ damage to my weapons. Since I happen to dual wield axes, that's 100+ armor and 20+ damage for the low cost of 6 ingots (one per piece improved).

tl;dr: do both. After a little bit, perks are easier to get, both skills are very very good, and the two skills really benefit one another nicely.

You can block with a two-handed weapon. You can't dual wield 2 two-handed weapons (because you don't have four hands) and you can't block while dual wielding one-handed weapons.

You can get a lot of armor rating with just a couple of Smithing perks as long as you also buy the appropriate armor skill perks. See also.

Enchanting isn't usually used for armor rating (as you get plenty between with smithing and armor perks). Instead, it's used for spell resists and damage. You need to make the call as to whether you need two enchants per item, and let that guide your perk spending. Don't forget ring and necklace slots.