1. Use the search function...this has been asked and answered several hundred times in this very forum.
2. In general this is a bad idea but it kind of depends on what the two specialties are. If its Med/Peds with IM or Peds as a backup, no problem (this more or less goes for all combined specialties). If you're talking about Rads with IM as a backup, not a really huge deal, especially since you need a prelim year anyway. If you're talking about Gen Surg or any Surg subspecialty with anything else (including any other surgical specialty) it can and likely will bit you in the ass.