Is it possible for a specific Account to exclude some requests from contract billing?

Let me give an example. There is an account which has a contract only for server support but we also support users. So, we need the use a prepaid contract for server support requests but to charge per request for user Support incidents.

Furthermore, we have accounts with two different support contracts. One of networking issues and one for systems issues. Is it possible to assign 2 different contracts to an account and to assign requests to each contract based on some criteria?

Thank you