I have determined that these two types have problems.
ISTp and ISFp do not talk. At all. We've spent three weeks with the in-laws so far, and I bet my husband has said 12 words to my father-in-law the whole time. And my father-in-law is infuriated by my husband's thriftiness (currently the fact that my husband refuses to buy a new lawnmower and will only look at used ones.) But oddly, his verbal complaint is that my husband is *lazy*, not cheap. And he's both, but it seems odd that laziness is getting the complaints when the father-in-law is a serial napper too. They both stay busy for the same amount of time, but my father-in-law spends his time tending to our yard (gardening is his favorite thing in the world) and my husband spends his time running around to garage sales and wherever looking for deals, and then fixing up stuff he buys that's broken and then selling it and earning money. My husbnad doesn't understand my FIL's focus on the yard, and my FIL doesn't understnad my husban'd interest in "junk".
I'm quite fond of both of them but I'm entirely unable to mediate.
For some reason I'm not edit my text, so I apologize for the typos.