Two separate networks on 1 cable modem

We are a small office currently running a regular network (9 wired, 2 wireless) and also using the "guest" feature (7 wireless) on a Linksys E2000. We have a switch to connect our wired equipment to the E2000. If "A" is our regular network, and "B" is our guest network, is it possible to have 2 routers on 1 cable modem and still prevent network "A" from seeing Network "B" and vice versa?? We are looking to do this because we will soon be exceeding the maximum guest users (10) allowed by the E2000. We have another Linksys router in storage, I think it is a WRT54G, that we would use if this is possible. Thanks for any suggestions provided.

log into the WRT54G and change the WAN to Static and assign it 192.168.1.254 (if available) and set the Gateway to 192.168.1.1
Then change the LAN IP to 192.168.5.1 and setup the wireless security for your Guests. Also make sure to disable that the router can be configured via wireless connection

connect the WAN port of the WRT54G to the LAN port of the E2000

log into the E2000 and add the WAN IP of the WRT54G (192.168.1.254) into the DMZ of the E2000