Diodes are not the way to go... They will rectify the audio (AC) signal
such that only 50% of the waveform will remain. This will sound very
bad... and is likely to upset the audio amplifier(s) in your radio(s).
Look instead for an audio output transmformer with a low impedance
primary and a center-taped, low impedance secondary. Say the Stancor
model TA-112. Your radio wants to 'see', and your PA horn is rated at
(probably) something like 4 - 16 ohms. So, connect radio #1 to one
half of the center-taped secondary, radio #2 to the other half, and the
PA horn to the primary.
This is a more expensive solution that the "diode", but isn't this
always the case when you want to do it right?

MW