I hope some more guys will jump in with their opinions, but my understanding is that starting out, it's a good idea to have just one bass drum pedal -- a single. Learn with one foot first.
When you're good with one foot you can think about going double bass.
I don't know if you can buy a separate, bigger bass drum pad for that set. The salesmen will have to answer that. You may have to go to a different model or a different brand to get a bass drum pad that can handle two bass drum pedals. I know some Yamaha
electronic sets can handle 2 pedals on the bass drum pad, but you may be in a substantially different price range with those.