Basically the simple answer is yes. On the router you use as your internal gateway set two default routes. One is probably already being set by IPCP over the ADSL line. So you will need to set the second one to have the same ad/metric as it. The router will then on a per destination basis load balance the lines. This means that if you start a single transfer it will go out one interface, and a second transfer will go out the other. If you want to load balance on a per packet basis, which means a single transfer will use both interfaces, you must disable the route cache.
Matt Newsom