I just saw the Acom 2S1, described as a transceiver commutator - i.e. allows the use of a single HF antenna into 2x transceivers.

Just what I need - but is this a common piece of kit that may be named differently, can it be made, etc?

Never heard of it but I imagine it has some way of keeping the RF isolated form the txcvr not being used.

A good quality coaxial relay would work, the ascom unit does offer some protection against the wrong rig being used though.