This is kind of a mind boggling combination. Family medicine is geared for outpatient treatment of adults/kids/OB. If you want to work on a med/psych unit--then I'd think you'd be better off with an IM/psych program. If you do family medicine/psych, well maybe you could work in a clinic and address the psych issues that PCPs see daily (depression, anxiety, some stuff beyond that). But that would waste most of the actual psych training and sounds boring.
I don't really know what people do with this combo, unless they live in rural areas and provide the absolute full range of care to everyone. For that, it seems ideal. Partly because without the psych residency part, you'd have no psych training at all, and you might be the only person for miles around. The family part covers almost all the rest of medicine! About all you couldn't do yourself would be surgery, anesthesia, rads, path, etc. But you would be spread very thin.