There are benefits and drawbacks to a close INTP-INTP relationship. One benefit is a general commonality in lifestyle due to both having the same type of introverted nature. In my experience, there's also interesting conversations and shared likes/interests/goals as well, which can be fun and even stimulate growth. Most of the drawbacks I've seen that could be generalized stem from the fact that INTPs are Ti leads. Two extremely opinionated, subjective (and therefore defensive), dispassionate people, who tend to disregard social norms and by their nature pick out errors and irrationalities and seek to disassemble them can end up hurting each other. Since correctness is more highly valued by Ti than emotional resonance, sometimes emotions are disregarded for the sake of asserting a point.
I think it's possible for an INTP relationship to work. In my experience, it takes both people having a certain kind of mindset, one that develops camaraderie from mutual disillusionment and refinement of ideas. But, just like a relationship between any type(s), the specifics of how it operates would depend on who the two individuals are.
