If you truly have two VFOs, as was common with separate matching
transmitters and receivers (such as the Drake 4 line, Heath SB series
and Collins S-line) and some older transceivers with external accessory
VFOs, then it is possible to have both VFOs combined into the LO port of
the mixer and receive two frequencies (on the same band) simultaneously.
I don't remember that the Collins, Drake, or Heathkit allowed
simultaneous reception without user mods.
They do not. They do have two VFOs, really, not just virtually by memory
switching. With user mods the Heathkit SB-301 and SB-401 can have dual
simultaneous receive (and transmit too, though no doubt illegal and not
a good idea even if it weren't illegal) for certain, because I have done
it. It is only my guess that Drake and Collins could also be similarly
modified.
The oscillator switching/sharing scheme of the Drake is somewhat
different than the Heathkit. With the Heathkit twins, when operating in
transceive mode using a single VFO for both the transmitter and
receiver, they also both used the same bandswitching oscillator and BFO
(I don't recall whether it was the ones in the receiver or in the
transmitter) , so your transmit and receive frequencies were definitely
the same. There were three cables between the rigs for VFO, BFO and I
think they called the other one HET. In the Drake twins I believe that
there was a "netting" adjustment of the BFO , which if not done properly
resulted in your transmit and receive frequencies not being exactly
identical, even when transceiving using only one VFO. The Drake had only
one cable called INJECTION. The BFOs in the transmitter and receiver
continued to operate independently when in transceive mode, thus the
need to "net" them.
The details of Collins S Line I do not know for certain, although I was
often told that my SB-301/401 was a green S line, designed by former
Collins engineers. Don't know if that is true.
DE N6KB
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