Q: Do I need to purchase the collection feature twice if I want to use it on both Decked Builder & Decked Builder HD and sync between my iPhone & iPad?
A: Yes. They are separated purchases. Users would have to purchase both if they want to use it on both devices. Apple doesn't seem to have the capability to merge in-app purchases across different apps. In the meantime, the iPhone and iPad versions are optimized differently, the iPad version allows you to search and build decks in one screen, speeding up the whole process. Testing is much nicer on the iPad too.