Meindert Sprang wrote:

(Original post deleted)

If the horn is 20 Watt and your battery is 12V, you'll need at least a 2 Amp
diode. Ask for a 50V/5A diode and you'll be fine. Higher specs on voltage
and/or current are fine too if that is what is available. The price
difference should be negligent. We're talking about a sub-$ component here.

Meindert

Err... How is a diode going to isolate one radio from the other, since
the output of each radio is essentially AC? A diode will isolate the
second radio from the first during one that radio's output half-cycle,
but not from the other half-cycle. If two diodes are used, one in each
radio's output, there would still be the "backfeeding" problem unless
both radios were tuned to the same channel and were feeding the speaker
(horn) in phase. What would be the point of doing that? Additionally,
whacking off a half-cycle from the radios would not only reduce the
audio power due to the impedance mismatch (itself a destructive issue
with some solid state circuits) but would probably sound like poo-poo. No?

Ed