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I prefer to mount the HTs with their own antennas.
This is unlikely to give good results, but perhaps you'll learn a lot from playing with it.
what is the best configuration (vertical/horizontal/oblique)
the required distance of the radios (3-4 meters adequate?)
Tx or Rx on the top?
I'd guess you'll need a vertical spacing of 20 to 50 feet, depending on the radio design and
how well it can handle strong signals off frequency. Without any filtering you may also
have transmitter phase noise appearing in the receiver passband.
That assumes that there is no RF current flowing on the wires connecting them together, which
is very difficult to achieve using built-in antennas (especially with vertical separation.) You may
end up mounting the lower HT upside down so the audio and control cables don't run along the
antenna.
Or a few hundred feet of horizontal separation might be adequate.
I'd suggest putting the RX antenna on top to maximize received signal strength to help overcome
the desense problems.