Depending on the router that you have, you might be to write a script to switch between the accounts.
I know that you can have multiple PPPoE accounts with the D-Link DSL2500U router, so it shouldn't be that difficult to write something to toggle between them.
The MikroTik routers really makes something like this very easy, where you simply press <E> in WinBox to enable the PPPoE connection and <D> to disable the other PPPoE connection
I've recently helped someone to get a script/scheduler going to switch between a 7-7 account and uncapped.
Mikrotik rocks. The router you are looking for is the 750 or 750G. Otherwise you could always use your pc to login (modem in bridged mode), just setup two pppoe accounts on your pc and switch connections when needed.