Ok, got it, and the good news i that it works for an infinite number of radios 
radio 1 (car 1) freq A : DCS RX A
radio 2 (car 2) freq A : DCS RX B
radio 3 and 4 (pit 1 and 2) have 3 channels
channel 1 to talk to radio 1 : freq A DCS TX A
channel 2 to talk to radio 2 : freq A DCS TX B
channel 3 to talk to radio 4 : freq A no DCS
the car radios never switch channels, they have only 1 and stay like that
the pit radios (3 and 4) are usually always on 3 to speak between themselves without interfering with the drivers, when a driver calls they can hear it and switch to the relevant channel (1 for car 1 and 2 for car 2) to reply.
Tested and it works…
the beauty is that we can add as many cars (or bikes or whatever) as we can add channels…
Thanks for the tips