My purely empirical / observational note is that activity relations work well when they're between aristocract introverts (thus, positivist introverts - ISTj / INFp and ISTp / INFj). Democratic introverts (thus, negativist introverts) will wear each other out, ISF types don't seem to be tailored for INT types - often these relationships might even last a lifetime, however the lack of spark / understanding is obvious. Democratic extraverts (positivist extraverts) become too wild when paired, furthermore they generally think of themselves as being the coolest shit around, thus they might naturally repel each other / sense that the other isn't complementary. Aristocratic extraverts (negativist extraverts) tend to become explosive when paired, relationships might last if they enjoy quarreling / have sadomasochistic tendencies, yet they're likely to lead to abuse on the long term.