Well there is no reason why a 2nd router won't route if setup properly. I'm assuming it is internal on the LAN. You'd also need a static route back from the main router (Tomato?). There's no need for MTU settings since you're dealing with wired ethernet, only the main WAN port is a concern and that usually takes care of itself.
If you going to go with a proper router you can do it for a lot less money with a Mikrotik box. Tiny little thing that costs no more than a basic consumer router. There's more to the config than even Tomato albeit a bit more to learn at first. »
routerboard.com/I use the 750G at home and a 450G at work, two WAN connections and lots of routing going on, no problem. (I also have a /29 subnet and get to use them all)