The easiest way to do this, probably, is to have one router for both DSL lines and have that router use some rules to determine where to send the packets. This is pretty easy to do with commercial-grade routers (CIscos and such), but probably impossible with consumer-grade ones. You might be able to do this with a PC running a quality OS but it's going to be more difficult than if you had a "real" router.

Here's a page on how to do source-based routing under Linux.