The part about communication sounds dubious. As far as I know, the HR-S04 or whatever it is called, is deaf unless it's triggered. An on board MCU controls its operating sequence. So it can't sit there listening for a signal without any sync. But if you have sync, it's by means of some other communication. So you've lost the point unless you can transmit a very wide bandwidth over the pulse channel. But since the on board micro on the module controls all the timing, only TRIG pulses can be emitted on command. The pulse characteristics are locked in the micro's firmware, you can not change them in order to communicate. I re-read the original post, I first thought it was about the usual ranging application but actually there is no explicit mention of that.
You could probably devise some means of communication this way, but due to the fixed nature of the hardware behaviour, it would be extremely slow.
Suppose you have GPS receivers on both ends. Now you can sync pulses. Perform the sequence I posted above. You could send a TRIG as often as possible. Unit B either sees a pulse right away or times out. That's a binary stream. Just very slow.
You would have the distance as a bonus... GPS to give a fuzzy view and the time, ultrasonic to correlate that with the inter device distance.