There are a couple of ways to do this.

1. Use resistive pads at the outputs of the two radios and a resistive
summation circuit to feed a separate amplifier which then feeds the
single speaker.

2. Use a stereo-to-monaural isolation transformer backwards. Each radio
feeds one stereo input and the monaural winding feeds your single speaker.

I am certain #1 will work, but it obviously requires purchase of an
amplifier and some soldering. Amps are not particularly expensive, but
this is probably not a job the average boater would undertake unassisted.

The second solution is more elegant, but I have no personal experience
with those transformers. Power rating would be an important
consideration, of course. It is my understanding that they provide on
the order of 20 dB of isolation, which should be more than adequate to
protect the two radios.

Good luck.

Chuck

Tamaroak wrote:
Two horns would be ugly. There has top be a way to do this.

Capt. Jeff