Crab, perhaps you are thinking of the "T/R+G" setting on your UHF/VHF box?
I concur with the other contributers in that as far as I know, this facility doesn't exist in the civvie world. Aircraft have either one VHF or two similar boxes, depending on their role / capability.
I think what you have highlighted is that the TDA is inadequate for an immediate and short term operation in Class G airspace. Even though we listen to 121.5 there is a good chance that we would miss a one-off broadcast because sod's law says we would be listening to another frequency at the time. (We normally DO revert to 121.5 for a "listen out" in transit on the "spare" box but only if we aren't already using it to talk to a minor airfield or listen to an ATIS etc, as we often need to do).