You cannot load balance two ADSL lines unless you run BGP -of which I am sure that is not available.
Another option: install a router that has three ethernet ports. One to the LAN and the other x-connected to the ADSL modems. Set a default route on the router to each ethernet port (for example):
ip route 0.0.0.0 0.0.0.0 e1
ip route 0.0.0.0 0.0.0.0 e2
The router will do per-destination routing in a round-robin fashion and the max throughput will be one ADSL link.
Good luck