While not directly answering your question, I used a Twin WAN Router at home for a while for a cable and wireless connection. It works decently, and allows you to specify certain ports to always be routed out a certain ISP. The only reason I recently discontinued use of it was due to plans to get away from the cable company, but still haven't fully done so yet. In fact, I may look to sell mine soon if you are interested.

In theory a Linux router could do this too, but I spent many hours trying with no complete success.