Peter wrote:
"---but don`t want two coaxes and antennas, so what is the simplest
method of joining the two aerial outputs together? I am assuming they
won`t like just being joined together."

Good assumption. Tom Bruhns and others have excellent answers. An r-f
switch is simple and effective. Switch the antenna and its cable between
transmitters.

There are duplexers which allow combining the outputs of multiple
transmitters to one antenna . They replace an antenna switch or relay
when multiple transmitters must have access to an antenna
suimultaneously as is the usual case of the usual audio and video
transmitters used in TV broadcasting. But, why bother with a duplexer
requiring tuned cavities, circuits, and, or, multiple lines if
simultaneous operation of the transmitters is not required?

In the case of multiple transmitters to be operated simultaneously, they
can get much isolation by end to end antenna stacking. The stacked
antennas fall into each other`s null zones and this isolates the
individual antennas much as does cross-polarization. I`ve seen estimates
of 1/10 as much coupling between end to end antennas as compared to side
by side antennas at the same distance from each other. This guess, 10
db, seems reasonable to me. Cross-polarization estimates I`ve seen for
coupling are 20 db down from coupling for same polarization of antennas.

Best regards, Richard Harrison, KB5WZI