Many people have done great with more than one qbank, but for the majority of people that I have talked to, they say its best to stick to ONE qbank and FA at the end of the day. Once you get those two down cold, you can move onto other qbanks.
I personally started out with kaplan qbank, did about 500 or so questions. While I think the qbank was good enough, it wasn't UW caliber in terms of questions AND explanations, so I dropped Kaplan and started going through UW. Maybe I might switch back at the end of my board study to drill minutia, but as of right now, UW x2 is where its at.