In the past, I somewhat successfully used dd-wrt to bridge 2 Linksys routers in the manner you're describing. It was in the pre-Roadshow days, however. I say somewhat because I found that if there was *any* other traffic on the bridged router (i.e., the one not connected to the main modem) the bridge connection would fail and every machine after the first to transmit would be effectively offline. I use static IPs.
I remember setup being incredibly difficult - half a dozen very similar settings that all sound like they would do what I wanted them to, but only one of them did, properly, consistently. And I can't recall the right one. dd-wrt's abysmal documentation didn't help the situation either. Can you maybe post some screencaps of your config?
I've since replaced my wireless infrastructure with last-generation Apple gear, which took about 3 minutes to configure. The boost to wireless N was a nice perk, too. You could probably get similar or older equipment for a decent price now.
