You actually can separate out the forward and reverse waves on a coax, but it requires good impedance match on both ends, and some transformer based magic-Ts. So you can do full duplex on a single coax, sharing the same bandwidth. It’s just not practical in long runs since you’ll only get 20 dB isolation between directions.

A common use is carrying Ethernet over coax when retrofitting analog CCTV cameras to IP cameras, and you don’t want to run new cable.