Occasionally I help residency applicants who are applying in two specialties. This tactic can be strategic in specific scenarios, but it also makes the process more complicated. Here are some quick clarifications to help:
1) Different personal statements can be assigned to different programs. This means you can assign a pediatrics essay to pediatrics programs and an internal medicine essay to internal medicine residencies. Of course, you need to have two versions of your statement, and you need to ensure you assign each correctly.2) Different letters of recommendation can be assigned to different residency programs as well. (A maximum of four letters may be assigned to each program.) You'll either need to have letter writers for different fields, letter writers who will craft two types of letters, generic letters (less compelling), or a mix of these approaches.
3) Assuming you are not applying in ob/gyn, EM, or their program variants (which require the ResidencyCAS), you will have one application, and it will go to all of your programs. So, for example, if you flesh out your membership in multiple pediatrics organizations in your ERAS activities, that list will be viewed by your internal medicine readers as well - and might make them question your commitment to IM.