Yeah, I think [MENTION=12050]TooShyToSay[/MENTION] is right that INFJs can figure out other INFJs, even if they can't figure out themselves. Two introverts also could provide the kind of intimacy without the threat of exposure that introvert/extrovert couples may face.
But, two INFJs together would each have Fe as their secondary function, meaning they would probably need each other to support themselves. This could work out beautifully, or it could work out awfully... Also, INFJs tend to hold those very close to them to the same perfectionist standards that they hold themselves to, and if those standards differ between two INFJs, then there could be trouble. Finally, while INFJs could probably understand one another and hopefully support each other's emotional needs, they also tend to gravitate or even obsess over a particular subject or theory, and if two INFJs get together but don't match in their interests, it could be difficult balancing the need to be together physically and the need to be apart intellectually.
When you're young and trying to figure out whether the person you are with is "the one", using MBTI might help you understand your own needs better, but be careful about using it to understand your partners', especially if they don't want to take the test honestly or don't really care about it. You should judge them based on who they are before what type they are. Then, if your boyfriend does not understand you and you feel you need to be understood, you probably already know the answer as to whether your current BF is "the one".
One other point: I'm nearly 40 years old now and I can say no one, not one person in my life has ever really understood me, no matter how much I tried and no matter how close I got to them. I think this may be part of the curse of being an INFJ: the constant need to be understood at a level but the inability to express yourself at that level. I have decided that I do not need complete understanding from those with me (and that took me a long time to decide) and I've found I'm happier for it. If I really couldn't be with anyone unless they really understood me, I'd probably be alone my whole life.