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 statement to internal medicine 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.
Thus, when you select a program to apply to, you will be able to choose which documents (personal statement, LORs) you would like send to that program. ERAS, in this way, allows you to keep your options open.