Sure, you can have two separate accounts if you want. There is no way you can mix and match shaped and unshaped bandwidth on the same account (therefore you would *have* to get two accounts to do this), but you can effectively have your "prepaid only" account (which costs nothing other than the bandwidth you buy) set to unshaped and use that to top up on.
The mixing and matching of prepaid and fixed-cap services is but one option, and is a pretty attractive option for a lot of users, as they can buy a certain amount of fixed-cap GB's (at less than the per GB rate), and then top up with any additional as they require.
In short... or is that in long... yes, yes you can do what you propose.