To implement
1) Interfaces > Ethernet > dmz, wan (both)
Uncheck "add default route ..."
2) Interfaces > PPPoE
Add your new PPPoE interface
Keep "add route ..."
3) Intrefaces > Interface groups
Make new group wans = wan1 + wan2pppoe
4) Replace wan to wans everywhere necessary
To revert, uncheck "add route ..." on PPPoE and enable "add default route ..." on wan