What's the path to the internet? Through the wireless router? What do you expect/want it to be?
If that's the case, you should just turn off all the "routing" functions of the grounded router. Plug it's uplink into the wireless router and then use it as a cheap (as in you already own it) switch/hub.
Another thing you could do is create two home networks. One for your "insecure" boxen and one for your "secure" boxen. But thats more work :)
Anyways, on to your problem. Each device should have it's own, unique MAC address (Unless you are doing something special, like failover). I think you'll need to be a little more specific about your setup. Perhaps some ascii art
Wireless router (192.168.1.1)
Redhat box (192.168.1.2) \
Grounded router (used as a hub)
OS X Winders
Or something like that.
edit: fonts seem to have a different idea of how big a space is. Oh well, I've already played with fonts this week, so I aint goin' fix it.
edit2: cut and paste from fixed font terminial window