I agree that if possible, do electives and one sub-I.
Sinai requires a sub-I in medicine, peds, ob/gyn, or gen surg, so if you're applying to anything other than that you end up doing 2 Sub-Is in different fields. I've heard of some schools that require a medicine sub-I regardless of field so you will be in good company with 2 Sub-Is if that's what ends up happening.