Scania, and probably others, are working on platooning trucks. From what I know it is not trivial, for example most of communication methods we have have too long round trip delays and are not 100% reliable so a backup is needed maybe in the form of autonomous breaking system.
If they are too far away they will not get the lower fuel consumption (due to air resitence), and they wouldn't need micro-second latency response times. So they could use the current radio technology if the antennas are too far away to touch. There could also be sensors in the antennas to feel if the vehicles are out of sync. And alert the driver when they are disconnected.