I have two new VHF radios, both of which have a PA and foghorn output
and will send a 20 watt signal into a horn mounted on my flybridge.
Standard Horizon says I can't just hook the two outputs together to the
horn because the one radio will backfeed into the other.

Their solution is to get two diodes and wire them on the + wire before
it gets to the horn. The other option would be to have two horns which
would be ugly.

Question: What kind, size, specifications do I use for said diode? The
guy at Radio Shack looked at me like I was from Mars when I asked him
about this.

Is there some other way to do this?

Capt. Jeff