As the previous poster stated it probably is unlikely that you will be able to complete another complete residency b/c residency is subsidized by Medicare. Medicare gives the hospital $100,000/resident/year for education, we get pennies for 80 hour work weeks. The length of time is dependent on what you match in first, Surgery would be 5 years and yours is 3 years. Any time over that would have to be paid by your institution.
Have you thought about the OB Fellowship? It's only 1 year (I think) and you will learn how to manage and deliver babies. Only caveat is that you are not allowed to perform gyn surgery and I am not sure about C-sections. It would be a great addition to your family medicine background.