Additional questions on using a second router as an access point

I have a linksys WRT-54g set up as an Access point, hard wired to my Cradlepoint router in my computer room. My question is about what channel(s) to use. I have the Cradlepoint on channel 11 and the linksys on 6. Another (fairly strong) system is nearby on channel 1, and weaker systems that I can see are on 4 and 8. Should I be using 2 channels(the access point on 6 and the main router on 11) or should they both be on the same channel? Looking at the Wifi Analyzer ap on my phone, it looks like the neighborhood is getting really crowded. What should I do if someone close starts using one of the channels I'n on now? It looks like someone has two systems using channel 4, but it's pretty weak looking in my location.