This is based on my experience with INFP SO. We've been together for 6 years. He's 9(w1?) and I'm 4w5. I'll try to keep every answer short so as to avoid rambling walls of text, but please feel free to ask for more clarifications.
When it’s working – What are the joys and positive aspects of these relationships?
- How compatible do you think these two types are in general?
I think they can be very compatible, as long as they share compatible values and life goals. Both types have strongly-held beliefs which tie directly into their perception of the self, so any change to those will be difficult, especially with the inherent difference in communication between the two types, which tend to need a lot of adjusting on both parties. However, I think that being idealists, both types will already share somewhat similar values in the beginning, making it easier.
- Why are they attracted to each other?
Both types are acutely aware of and fully respect each other's inner space. The INF inner realm is sacred to the owner, and both types realize and appreciate this fact. When they find each other sometimes it can feel like finding a secure sanctuary of 'us' against the rest of the busy, complicated world, that neither truly understands and is comfortable in.
- How to they complement each other?
To put it simply, the J can give the P a bit of a direction, and the P teaches the J to slow down and enjoy the journey. This is a bit unnatural for both parties, but success in doing so will open up a richer, more fulfilling world for both. A lot of communication is needed though and communication must be open and honest.
- How well do they understand each other and why?
This is a strange one. I've said it many times before and I'll say it again here: as an INFJ, looking at an INFP is like looking in my mirror image. There's that comforting sense of similarity, but when you look closely everything is the total opposite, so the other party ends up being both familiar and foreign at the same time. INFPs and INFJs often arrive at the same conclusion, through different ways, and as long as they can both look at the destination and not the method, they will find it easier to settle arguments.
- What are they like together raising children?
No idea on this one!
When it’s not working – What are the challenges when two people of this type are in a relationship?
- What are some of the communication challenges they can have?
Fi seems to have a thing about maintaining autonomy, while Fe seeks teamwork. This can prove to be a big challenge in communication. For Fi, the Fe give-and-take approach usually feels manipulative and obtrusive, and for Fe, the Fi everyone-is-free approach usually feels selfish and uncommitted. The funny thing is that both parties usually want the same thing, but the way they go about it is completely different. The worst thing is, the other party's approach will often seem completely wrong to you, even if the outcome is the same, so you can both get stuck in arguing about 'why your thinking is so messed up', which can get very ugly and will only escalate conflicts.
- What are the biggest frustrations between these two types?
Forgetting how different your approaches are and forgetting to tune your communication accordingly. Then you end up fighting over the smallest, silliest things, and when it goes down to the minute interpretation of everything, it will get really messy.
One thing I will share is that since your thought processes are so different, you will never actually be 'right' or 'wrong' about anything. It might be a good idea to realize this from the beginning. All you can do is realize that something is hurtful to the other party and respect that, even when you don't understand why. Then you learn not to hurt each other's feelings.
- How can they take each other for granted?
I don't think they have the tendency to take each other for granted, because feelings are of great value, and both types can sense that something is wrong almost immediately. As long as you keep open communication and discuss feelings immediately and don't hold secret grudges, things will be all right.
- What happens with things “go wrong†between these two types?
Both types can be extremely stubborn. They both need to hold true to their values as this is a great part of their identity. So when you start to go down the 'who is right/wrong' route, things will only get worse. There is no right and wrong. You're operating in completely different modes of judgment, so you can never arrive at that. If neither sees it and tries to put the right/wrong value to any conflict, it can blow up way out of proportions and bring out all your differences. Then the foreign internal landscape that you glimpse in the other person will scare you and shake your trust, and you feel that there's this uncrossable abyss between you and you give up.
Advice for couples – What recommendations do you have?
- What things should each type do to facilitate better communication?
Both types: try never to lash out. Both of you are susceptible to bad atmosphere and extremely touchy about word choice and tone of voice. Come up with 'safe' sessions where you agree to discuss things calmly, try not to take things personally, lay no blame and assume no intentions, and, of course, agree to not judge.
Again, establish that no one is ever wrong in any argument. Apologize for hurting each other's feelings, learn and move on.
- What advice do you have for each of the two types?
INFJ: give your INFP a lot of space. Don't try to fix everything (most of the time there's nothing wrong -- especially when the INFP is moody -- just let them be. It's not your fault and you can't fix it)
INFP: ask directly for things you want (in a tactful manner -- usually when it comes to this the INFP has already built up so much grudge and it comes out in full Te mode, which the INFJ does not take very well)
The most important starting point:
I read somewhere about the pitfalls of these two types that "the other's viewpoint/approach will always seem inferior to yours". This is the WRONG way to go about it. If you ever feel this, the relationship falls apart. Trust. Your INFX's inner world is as beautiful and pristine as yours. Respect it. Appreciate it. There are wonderful, refreshing things to see from their point of view.