|From the movie Before Sunrise.|
There's this theory that MBTI types have specific kind of relationship models (Read: problems) between individuals of different types. The one that often gets praised as the ultimate dream come true is the Dual relationships (No problems). That would be the type that has the same cognitive functions as you but in the opposite order.
So, when I was still new to things I pretty much just ignored this whole theory, because I didn't find it applicable in my own life. I'm an INFJ and I had never gotten along with an ESTP especially well. (If you don't count my brother, but you know, that's my brother, and I generally get along with all of my family.)
I also found that as much as some of the relationship types did describe well the relationships I had with individuals of that type, just as much of it didn't. For example, I know so many ISFJ's it's no wonder I have this Fe-aux bond with some of them, but then there are so many others I don't connect with like that, and even ISFJ's to whom I've been more like the benefactor I'm supposed to be to an INTP.
So, at this point I think I've formed somewhat more well-informed opinion about this theory. I don't think it's completely useless and without truth in it, but I also don't think it's the Holy Grail of cognitive function theories that some make it out to be.
In short, I think there are WAY more important things to be considered in a partner, and factors that play a much bigger role in why certain people form a strong, healthy relationship, and some don't, the most relevant (but not in any way the only factors) to my point here being:
1. The maturity level of the people involved and whether these match.
2. Enneagram + MBTI combination.
Let's say you're a healthy level (Ennegram level 1 or 2) individual INFP who just can't find the energy to sympathize with average Enneagram 6 struggles on a daily basis. Your Dual partner is an ESTJ, that's supposed to be a match made in heaven. You look for an ESTJ, but this person turns out to be an average health level (Let's say 4 or 5) Ennegram 6. There's almost no doubt that they're not your ideal partner (and we aren't even counting things like worldview and interests).
Okay, so this is totally simplified to illustrate the idea. But these are things that I have found to weigh so much more than the Duality theory. However, like I said, it's not completely without truth in it, and I think this is why:
Average maturity level individuals (Ennegram average health level or MBTI types that primarily use only their dominant and auxiliary functions) benefit most from the Dual relationship. To these individuals it may indeed seem like magic (Before Sunrise style), like you've found your other half, because functionally you have. You have little conscious experience with these functions (Like, average INFP with Si and Te, and average ESTJ with Ne and Fi). But it still resonates in you when you see someone else use these functions well. So they do complement each other.
Of course I still think they have to personally get along with the individual's Enneagram type to feel that connection.
(And I still think there is a lot more individual background, life choice, temperament, interest stuff that plays into it.)
But the reason this Dual theory is relevant and has some truth to it, is this, I think.
When it's individuals who have matured enough to use all their four functions consciously, I don't think there's much magic in Duality. Because these individuals don't necessarily need guidance for these weaker functions, or at least don't dream of someone to do these functions for them, because they already know these functions are in them, too.
And so, the healthier the Enneagram type is, and the more mature the MBTI type is, the more they begin to look like each other. And then, it makes little difference whether their partner is supposedly a Dual, a Benefactor, a Quasi-Identical, or anything else, because they don't need someone to complement them, to provide for them, or to mirror them, or anything like that. Because all these differences start to disappear, the more mature the individuals in question are.
(This still doesn't in any way mean that more mature people are more likely to find a good partner, because they still need a person with similar maturity level to be content in the relationship, and quite frankly, the more mature you are, the fewer your options become.)
So, even though these relationship models might ring more true at average levels, I still don't think they are a good way to try to determine whether someone might be a good partner for you. People are always more than the sum of their parts. There is no answer that could definitely tell you before hand, whether someone is good for you or not. (Even if I think the Enneagram+MBTI combination and maturity level together are a much better indicator, once you know yourself well enough, you wouldn't know these things without getting to know the person at least a little deeper, so it's kind of a retrospective interpretive tool.)
I've had really meaningful and successful relationships with individuals of types that were doomed according to this relationship theory, and really bad relationships with individuals I should have gotten along with according to it. And I still have yet to find an ESTP who would sweep me of my feet, or even become a long term friend. And it's not for the lack of ESTPs in the world.
That's just my two cents.