What does that mean? I have heard it quite often, what would be perfect about a double? I am referring to a 2 meter FM QSO where 2 operators transmit simultaneously and both are heard clearly. Wouldn't that always be the case?
  No. It is rather rare. It is the nature of the "Frequency Modulation" system, that unless two different signals are almost the same level one will be received and the other ignored completely.
  If there is flutter (QSB - which frequently is the case with mobile signals), the result can be switching back and forth between them, a swishing sound sometimes results, depending on the relative carrier levels and what was said.
  With AM signals (Amplitude Modulation) there is normally an addition of the two signals, blended - mixed together) as though the two voices were
using the same microphone. *
  With FM an effect called "capture Ratio" is the rule. You really can't follow the frequency of two signals with one circuit.
  The "Capture Ratio" is a function of the design of the FM detector circuitry. It converts the frequency of the incoming carrier into a d.c.
voltage, if the carrier is modulated, changed by audio (voice) it converts it into the original audio, (voice) - FM demodulation.
  Several FM detector designs are supposedly improvements over the previo us designs, each have a different compromise of qualities (like everything else in life) and "Capture Ratio" is one value that varies.  More on this subject will be found in the `Amateur Radio Handbook', our bible.

* AM is still used a lot, for the .55 to 1.650 mHz "Standard Broadcast" radio stations and on air planes, I always wandered why planes didn't switch
to FM, we HAMS love FM it is so clear and noise free, but the `double' ability of A.M. is very much desired. If a plane is in trouble,the calls for help can be heard, along with the signals of nearer planes (to the airport) even if it is in-between airports.
  If two AM signals are not on the same frequency, the difference will produce a tone, the frequency will be the difference between the two carrier requencies. With AM radio stations, the (FCC) frequency tolerance permits the carriers to be no more than 20 cycles apart and the human hearing normally is un-responsive below 20 cycles.