Here are some clarifications about what ERAS offers that will help you apply in two specialties:
1) Different personal statements can be assigned to different programs. This means you can assign your ob/gyn essay to ob/gyn programs and your internal medicine (IM) statement to IM residencies.
2) Different letters of recommendation (LORs) can be assigned to different residency programs as well. (A maximum of four letters may be assigned to each program.)
3) You will have only one MyEras application. It will go to all of your programs. Note that if you list your membership in multiple ob/gyn organizations, that will be viewed by your IM readers as well (and might make them question your commitment to IM).