If I'm correct Helm will do this for you. It can control multi servers from one interface. I still think each servers will have different Ips and Helm will link them together and you can manage your accounts on both machines. But I could be dead wrong.
I believe you can also do that with Plesk, if purchasing their clustering software. All in all, if just for shared hosting, most of the billing modules, have the ability to split up hosting accounts to add in either rotation, or until one is full per your specs. In either case though, I believe for your setup, you would be using individual IP's for each server then.