Hi Peter

Use a 1/4 wavelength 75ohm coax from each TX joined at a T then 50 ohms to
the antenna. The same idea is used to join two power amps in parallel.

This assume the back end of the TX can handle some signal from the other TX.
Wont be all that much there though. Work it out.

Note this assumes 50 ohm output Z on the TX's

I have two small 433MHz transmitter kits that I use for some remote
switching. Only one is on at a time. I want to run a coax from them to
a roof mounted aerial but don't want two coaxes and two aerials so
what is the simplest method of joining the two aerial outputs
together? I am assuming that they won't like just being directly
connected together.