Well, the wan and port 1 on the router are interchangeable, so by using one generally you can't use the other; so plug the router into your other router via port 1 and use the other ports as switching ports. You'll want to be sure you're only running DHCP on the router which will be your primary.
I'm doing the same thing here btw, my main is a WRT54GS v1.1 and my secondary is a WRT54G v5 (and my third is a vonage voice adapter)
