>>just to make sure: when you say "full-track-mono heads", do you mean one track?<<
Yes, I do - you could still bounce with another tape machine and a mixing console though
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the great advantage of the full-track-mono was that it uses (nearly) the whole width of the 1/4 inch tape, getting the most of dynamic range and saturation out of the tape.
the 2-track-mono, as Green Lady describes, would either use less than half of the 1/4" tape - and you could reverse the tape and use the other side, or...
use both, in one direction, leaving an unused lane in the middle wich could produce a lot of hiss if the tape was played back on a different machine.