I haven't heard of combined OB/Genetics residency, but I do know of people completing an OB residency and then pursuing a 2 year residency in medical genetics (look at NIH's residency in genetics in Washington, DC as an example).
With respect to MFM/Genetics, the genetics component, when taken with MFM is more geared toward Women's Health than the traditional Medical Genetics residencies, so be sure to speak with the programs about their specific objectives and aims prior to committing to one.
Other specialties that will give you a chance at pursuing genetics (purely from personal experience) are Pathology (1 additional year), and Pediatrics (2-3 additional years)
Hope this helps ya! Good luck!