Me and my twin fraternal brother are both INFJ's.
And I should say - there are a bunch of positives and negatives about our relationship.
Positives:
1) We are mind readers. Well, practically.
I can look at him a certain way and he can look at me a certain way, and we can instantly know what we're talking about. This happens on a daily basis. Sometimes people around us will ask us what we're doing, and we'll both translate the non-verbal communication back, almost in an identical way.
2) Whenever I am feeling something incredibly complex and cannot articulate it, he can just look at me, smile, and say, "I understand." I do the same thing to him. I guess because we both operate on extreme feelings, we understand the need to "get it out" and the frustration of not being understood after these feelings are revealed.
3) He does his thing, I do mine. We recognize the need for alone time. Sometimes we'll spend entire days in different rooms, group up at the end of the day, and my mom will look at us and say, "did you guys have a good day together?" and we'll both smile and say, "yes! perfect." At the same time, sometimes we have days where we are completely inseparable. He'll want to constantly hold my hand or be really huggy, and I'll respond the same way.
Negatives:
1) When we have emotional outbursts, which happens from time to time, we respond to each other aggressively. Not with anger, but with confusion, like "where did this come from?"
And we respond by mirroring each other. When he is frustrated or sad, I get frustrated and sad. When he gets reclusive, I get reclusive. Our negativity feeds off each other. We typically need someone else to intervene and "pull us out" so to speak. That's probably why we have a mutual friend that's an ESFP - he usually comes around and gets us out of our muck-rut.
2) When we're apart for a long time from each other, we get drained much faster - I think because we have a tendency to surround ourselves with E people to counteract our I-ness. Being together is rejuvenating, being apart is sapping. Plus the fact that we feel, most of the time, the only people that understand us is each other... which leads to frustrations with our friends and s/o's. It's kind of fatalistic thinking when you have someone there who gets you completely and everyone else doesn't come close.
3) We both try to be more P and less J with each other, which leads to alot of debate about the dumbest stuff. Like, where we want to eat dinner. He'll say, "anywhere you want to go" and I'll say the same thing, but in the end we both want to go to different places and then we'll get stubborn about it and have an argument. That's typically when our P friends come in and tell us we can do both... haha.
Weird things:
1) EMPATHY BATTLES! We don't operate on fighting - we operate through giving. But if he out gives me, I'll get frustrated and angry and then give him something back or do something that's bigger and better. It gets a little out of control sometimes. It's not typically material objects - usually it's taking people out to dinner or cooking or cleaning or gestures of affection... but it's annoying when you have someone around always trying to outdo you to be the "better" INFJ.
It's like one big passive-aggressive, warm-fuzzy-feeling battle.