That's a very good question, and it's something that I haven't considered before. I think for now we'd like to try and keep it to one account per user, just so that everyone has a fair chance at a slot in this campaign (since the rates are quite high), but no, there are no rules in place right now that prevent users from enrolling two or more accounts at a time. It's something that I'll have to look at in the near future if I sense that a select few users are filling the campaign with a few of their accounts each, preventing others from enrolling their Twitter accounts in the campaign.