Earlier quoted context omitted.
> I used to be a controller at Los Angeles Center in the 90's, and worked Aspen SR-71's (and later under the NASA callsign when they flew them out of Edwards AFB). They could hear ATC but not other aircraft, because civilian aircraft are on VHF frequencies, military on UHF, and anybody above 60,000 feet (like the SR-71's) were on a separate, center-wide UHF frequency so they wouldn't have to switch frequencies consta…
I imagine they have multiple receivers and can listen to any number of frequencies at once, but as a pilot you won't be listening to civilian frequencies....because why would you? It's just noise that's getting in the way of the mission.
This seems a perfectly sensible thing for the RSO to do on a training mission, since on a real mission he might need to monitor multiple frequencies:
> I was beginning to feel a bit sorry for Walter in the back seat. There he was, with no really good view of the incredible sights before us, tasked with monitoring four different radios. This was good practice for him for when we began flying real missions, when a priority transmission from headquarters could be vital
At this point, they have passed their hours threshold and are heading back home. The pilot says:
> Just to get a sense of what Walt had to contend with, I pulled the radio toggle switches and monitored the frequencies along with him.