How to achieve a 2 way ensured lucky trade where 1 of the traders has already 9 luckys obtained through trading?

I plan to make a trade with someone. That person has a shiny magicarp, I would like to have as a lucky form, the other person wants one of my shiny moltress as lucky form.

The problem is, my trade partner has already obtained 9 lucky pokemon. But I know, one of my first lucky trades I made with someone, that person had aswell already more than 10 luckys, but since I didn't had them, it worked for us both getting a lucky from the trade anyways. I want to use that here for my advantage. So we plan to trade one of my pre 09/2016 pokemon for the other persons magicarp and one of the other persons pre 09/2016 pokemon for my Moltress. The problem is, I can't remember, if it was my pokemon having been from pre august when I made this trade back then and it worked, or it was one of the other person.

So my question is:

Who has to trade in first his pre September 2016 caught pokemon for our plan to work out? Or isn't it important at all, as long one of us hasn't obtained overall 10 lucky pokemon yet?

Clarification:

I am not asking for how to improve the chances of getting a lucky pokemon or something that alike.

I am asking, if it is relevant, who trades his pre September/2016 pokemon in first, to obtain the 100% lucky benefit in the specific scenario I described.

The person who trades the pre 09/2016 Pokemon must have less than 10 lucky Pokemon to guarantee a 100% lucky rate.

Regarding your proposed trades with your friend:

• Your trade partner, as he has less than 10 (9), can trade a pre 09/2016 Pokemon for a guaranteed lucky Pokemon, though sadly his shiny Magikarp won't be pre 09/2016 (shinies weren't available then). If he doesn't trade a pre 09/2016, then trading one of your 09/2016 will only guarantee it if you have less than 10 luckies
• Him trading one of his pre 09/2016 Pokemon for your Moltres will work. Note he will then have 10 lucky Pokemon. and him trading with a pre 09/2016 Pokemon again no longer guarantees a lucky Pokemon.