Note that in 2.4GHz, the channels overlap. So if you have one device on channel 1 and the other on channel 2, you're right that they're interfering. Channels 1, 6, and 11 are far enough apart from each other that they don't overlap with each other, so make sure you're using channels that are "5 channels apart" like that to avoid overlap.
If you have both devices using the same SSID, make sure they're using the same security mode and wireless password as well. Also note that client devices can optimize their roaming/scanning algorithms if the SSID is being broadcast, as opposed to a hidden/closed/non-broadcast SSID.
For best range and performance, leave the AirPort Extreme in its default radio mode of doing b/g/n in 2.4GHz and a/n in 5GHz. I've seen people turn off the legacy a/b/g protocols because they didn't realize that even N clients fall back to those rates when they need greater range, and I've seen other people turn off N in the 2.4GHz band really for no good reason at all, not realizing that their dual-band N clients can get a little better range by switching from 5GHz to 2.4GHz at distance.
Edited to add: Since your Verizon FiOS box has to be at the head of your network (since it's the only device with the MoCA coax port that connects to your FiOS demarc), let it do NAT and DHCP for your whole network, and make sure you turn those services OFF on your AirPort Extreme (i.e. put the AirPort Extreme in "Bridge mode").