I have both cable and dsl service. The router attached to the cable service is 192.168.0.1 and the router attached to the dsl service is 192.168.0.254. I determine which service my computers communicate through by setting the gateway assignment to either of those addresses and it just works.
I can connect to my computers from the outside by port forwarding from my routers to the computer that I want to connect to.
However, the outside ssh connection works only if the computer's gateway assignment matches the router that I'm connecting to. I suspect that I have to set up some sort of routing assignment to tell the computer to reply to the gateway that the ssh connection request came in on if it doesn't match the current gateway assignment, but I have no idea how to do that.