I have an ADSL modem (Actually billion router) that runs in bridged mode, i.e., does not establish any PPPoE connections itself. Connected to that I have a linux box with two NICs. The linux box makes two PPPoE connections via the Billion modem at the same time.
Then I set up routing rules (thanks to antib0dy for that) that route connections to local IP addresses via a capped IS account (local only) while all other traffic (i.e., international) goes via SAIX. Basically I use the IS account's international portion until capped, and after I am capped I start the second PPPoE to SAIX's prepaid account to ensure I still get international connectivity.
After the linux box, I have a 4-port WLAN router which handles the lan and routes all connections to the linux box. So actually linux box is simply an additional step betwen the lan and the bridged-mode modem so it does not matter whether user devices are connected via cable or WLAN.