It would be intense and passionate for a short time. What happened after the initial honeymoon period would depend on whether the relationship was solid or just built on each person's overly idealized image of the other. INFPs have to be careful about falling into that pitfall of idealizing the other person because when that person fails to meet those ideals or standards, it can bring the INFP hurling down to earth from their fantasy. Now take two INFPs and imagine the great potential for disappointment on either side.
This is exactly what happened to my relationship. I know this thread is kinda old but I just wanted to say from my experience, unless both INFPs are mature, know what they want from the relationship, and not afraid to speak up when things aren't going well, then don't do it. You're just setting yourself up for a loop of frustration and passive aggressiveness. It was my first relationship, so just imagine how much more you would idealize your first love when you're already an idealist. We both did it and we both knew it wasn't working but we didn't admit it to each other until I confronted her about something small because I was tired, and then all this stuff came out, then break up...I also felt like she could be very self-centered. She would never show any interest or respect for hobbies or interests I held dear, she would always complain if I wanted to show her something & when I called her out on it, she would say she was just joking, but it would hurt my feelings because I always try to be open-minded about other people's ideas & interests, whether I like them or not. And since she would always write off my interests as weird (I mean, I am weird, most INFPs are to a certain degree

) I felt like I could never have an intellectually stimulating conversation with her. It's like she didn't have an opinion on anything. She didn't like deep conversations which is what I crave. I would try and go out of my way to find something she liked and could get into but it's almost like, if I suggested it, she'd write it off as weird and not give it a chance. I had her take the test early on in the relationship and she got INFP but I thought there had been a mistake because of the lack of abstract, deep thought and the lack of openness to new ideas, so I thought she was an ISFP but when I researched the ISFPs, it just didn't seem like her either. I guess we're just different INFPs. But I just couldn't stand how she would joke about almost everything or write something off as weird. While I love to laugh and have fun, I need someone I can also be serious with and enjoy intelligent conversations with. Another problem I had was that she didn't have any ambition or goals. It's as if she didn't care about anything and was just plain lazy. It frustrated me to no end but I felt I couldn't call her out on it because while I did have goals/ambition, I had nothing to show for them because I was also pretty lazy and hadn't accomplished much.
I just don't think it's a good idea to date someone so similar to you. It's good to date a type/someone who complements you, so if you're weak in one area, they can be the strong one. I feel like I was looking for an ENFJ in her, someone who was nurturing and supportive, someone who loved to theorize and think abstractly as well as someone who's ambitious and knows what they want to do with their life that way they can help me get into motion. I think a big thing for me is that I wanted to be inspired & I didn't feel that at all in this relationship. It was good at the high of relationship but once we started getting to know each other, everything just came crashing down. I'm not saying that she's to blame for everything. I'm sure she blames me for not being enough this or being too much that. It just wasn't a good combination.
And I do have to be honest. I felt from the start that it might not work but I didn't want to believe it because I was so eager to be in a relationship. Don't be like me. And don't be too afraid to voice your concerns or issues. Listen to your Te and do what you know you should do because it'll save your Fi a lot less turmoil in the end.