I'm actually surprised that so few rigs since the Corsair/Corsair II, including their successors, have had that feature, in one form or another. It does come in handy when you're working split and trying to find the calling station!
Or receiving two separate frequencies in-band.
Imagine the possibilities.
One can listen to the phun on 14.275 and 313 at the same time. Or 7.185 and 7.200. Or 3.840 and 3.843 - you get the idea...
"Everyone wants to be an AM Gangsta until it's time to start doing AM Gangsta shit."