You can only book fastpasses for 1 park initially. So say you wanted to go to Epcot and HS. You can only book fastpasses at only one of the 2 Parks initially. For example, you could get a tier 1 and 2 tier 2s at Epcot and use them. Then you could grab a tier 1 at HS(maybe hard to get day of) as well.
Alternatively, you can choose to only book a tier 1 at Epcot and skip the tier 2s. Once you use that tier 1 at Epcot you can try to get a tier 1 at HS.