Making it truly combined is probably out of the question now. I wouldn't even ask. I would go at this a different way. Maybe you can let your psych PD know that you have possible interests in persueing a career course that puts you on the psych/IM forefront. If you ask, maybe you can have a majority of your primary care months be IM related.
Assuming this works, maybe talk to the IM program director around PGY-3. Ask him/her if it might be possible to continue as an IM resident after completion of your program. If he/she is agreeable, talk to your psych PD about what you can do about getting your 4th year as close to IM as possible.
Ideally with your intern primary care experiences and 4th year experiences, maybe you can shave off some quality time on an IM residency.
I would also consider doing it this way because dual-residency programs are useless (my opinion), and it gives you time to reconsider. Psych has better hours and better pay than IM so odds are you will go into practice in psych - forgetting IM. I strongly considered psych/FM as well as psych/pedi/child psych and spoke to a few people that did the combined residencies. Every one of them quit FM/peds and focused solely on psych stuff.
I'm just saying to think about this long and hard before making any decisions.