I don't know the answer, but I must admit, that is a very good question. I can only say that having used HF for all of my career ( over 30 years), I would say more than one aircraft can transmit at a time. For example, lets say Jakarta and Brisbane are both using 13887(I made that freq up) ...in my experience you can have aircraft talking to Brisbane and Jakarta at the same time. But I could be very wrong.