Met someone along the way that was a PGY4 in psych applying for FM. Their best bet was with their home program (I'm not sure in the end if that's what they went with). They spent a lot of their elective time on IM and outpatient medicine (I think also OB and Peds), to re-introduce themselves to the medicine they hadn't seen in 3 yrs.
They said that it really varied program to program how much credit they were given, but it tended to be between 6 mos and 1 yr. I think they said that their home program offered them a full year. Its something you'd have to ask programs along the way. You'll have to apply, and I imagine you'll have to have some strong reason you're going for FM training (not just that you miss medicine, but some sort of plan of how you'll utilize both sets of training uniquely).
Alternatively, I believe there's also PC psych fellowships which will train you better at handling the medically complicated psychiatric patients as well as in other roles like CL, etc. If you're really interested, I'd check out the Association for Medicine and Psychiatry. They have some info on their website about combined programs and about fellowships in each field to better prepare you for the other. Below is a link, but I think there are even more:
The Association of Medicine and Psychiatry » Fellowships Unfortunately not the most up-to-date as most links point to nowhere, but maybe it's a start.
UWash:
Psychiatry in Primary Care Fellowship
NeoMed:
https://www.psychiatry.org/File Library/Psychiatrists/Meetings/IPS-TheMentalHealthServicesConference/2016/virtual-registration-bag/NEO-Medical-University.pdf
CWRU:
Public and Community Psychiatry Fellowship
Psychosomatic/CL Fellowships:
U.S. C-L Fellowship Training Programs