I'm still having trouble with this. I have double and triple checked that I am using the correct frequencies.    
If  I have the main on green preset 6 and use red preset 1 (freq 255.50)  for Initial, I never get any wingman responses from any of my commands.   
 If  I have Main on red preset 4 and Initial on green preset 1 (again, freq  255.50), I can get a few wingman responses once on station from commands such as radio  on, ECM on. However, all engagement commands do not work (no responses  at all), nor does the RTB command which I have needed to use when  wingmen call bingo.        
The mission data card says use Initial for wingmen commands. What am I missing?