PPPoE Pools and Gateway

Suppose I have two different upsteams to the Internet.

RouterA 12.1.1.1/24

RouterB 208.1.1.1/24

RouterC-PPPoE server talks to A & B on one interface and PPPoE users on another.

I want to assign some PPPoE users IP’s that are and come from RouterA and some that come from B. They are seperate paths to the Internet actually and not balanced at all. The plan is eventually RouterA will go away when I finally get everyone moved over B over like a 30 day window.

How would I do this? Would it be done with prefered source?

Matt