-----An INFP-ISTJ relationship can certainly succeed, but such a type combination is anything but a shortcut to personal growth. First, take a look at the charts, below.
-----Our inferior function is often tied to attraction--because it is the function/part of us we start off feeling like we must deny, so we tend to look for it in others to balance us. This search ensues because in order for our dominant function to be the dominant function, its opposing function must be, to some degree, suppressed. Here are the opposing function pairs: Fi/Te, Fe/Ti, Ne/Si, Ni/Se. Early in life, INFPs seek out relationships with ISTJs (Si-Te) and ESTJs (Te-Si) in particular because we are essentially using their conscious functions as crutches for our undeveloped unconscious (and vice versa). In the charts above, every use of one person's dominant or auxiliary function triggers the other person's inferior and/or tertiary functions.
-----Later in life, once we have integrated (and accepted) our tertiary and inferior functions--becoming a complete person in ourselves--then INFPs tend to seek out relationships with complementary partners. Here are the complementary function pairs: Fi/Fe, Te/Ti, Ne/Ni, Se/Si. So, a little later in life, somewhere between 25-35 I'd wager, unattached INFPs tend to seek out relationships with ENFJs (Fe-Ni), INFJs (Ni-Fe)--and other INFPs (Fi-Ne). Notice that in the chart below, the inferior and tertiary functions of both people are relegated to the shadow portions of the other. Theoretically, anyway, this coupling minimizes miscommunication--so long as both people are reasonably mature and have integrated their tertiary and inferior functions.
-----The following article says this and more:
INFJ, INFP, INTP, INTJ Relationships, Compatibility, & the Inferior Function | Personality Junkie
-----Unfortunately, the process of integration and accepting oneself is made more difficult in a "shadow" relationship--not easier. That's because to each person the other represents the eruption of a repressed unconscious. In short, it will take a lot of work to make the relationship work, but it can work.
Original Thread: INFP+ISTJ: mutual aid in personal growth?