You only need the control channel to be able to receive the site, but keep in mind that the frequency could possibly change to one of the others assigned to that site.
As far as being within range of two sites, I have programmed "base station" scanners for folks who like to listen primarily to fire and EMS. Since their location is within monitoring range of four sites, I programmed everything they want into one bank, using the nearest site. Then programmed the identical set of talkgroups using a different site into a different bank on the scanner. When the scanner goes quiet, they look at the signal strength meter. If there are no bars showing, they just push the button to start scanning the "back-up bank," until the primary site comes back on the air.
Just a thought. You may find something that works better for you, or someone else may chime in with a different idea.
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