Thanks, guys! Just to clarify:

What I envision are two separate setups. One downstairs, consisting of a MX-880 remote, MRF-350 base station and RFX-250 antenna, hooked up to a home theater system. Then a second MX-880, MRF-350 and RX-250 hooked up to a TV/cable box upstairs.

It sounds like both setups should be on 418mhz and I'd be fine, correct? Even if a person upstairs and downstairs are pushing buttons at the same time, this won't cause confusion/interference issues if they are both on 418mhz?

Thanks again!