Are you intending to add another NIC for the cable connection, or are you going to utilize the existing NIC for this traffic? It is possible to use the same NIC for both I imagine, although I have to admit I've never tried it. Are you going to use a switch or something to connect your DSL and cable connections to your system?
What I am thinking is defining a virtual interface on the existing NIC, and then sending the traffic to the cable modem via that interface.
I recall working with someone on something similar not long ago here. In that situation, the system was using two interfaces to connect to the corresponding WAN connections, but the concept would be the same. They were using a squid server and wanted to send traffic via a secondary WAN interface.
I'll see if I can track down that tread.
That problem looks similar to yours.