Years ago I had to do an install on a local PD's chief's car that as one of the specs was that we couldn't mount any visible speakers for the two radios. As this car had come without a factory radio we decided that we would utilize the space in the dash where the factory radio's speaker would normally be installed. Unfortunately there wasn't enough room available for us to mount a pair of decent sized speakers there we had to come up with a solution that would allow us to just use one speaker without using a lot of fancy electronics like a mixer.
What we came up with was an audio transformer that had an 8ohm input and 2 8ohm outputs. We simply reversed the connections, made the outputs inputs and the input the output. To make a master volume control we wired an 8pohm "L-Pad" between the tranformer and the speaker and the radio volume controls adjusted which one would be the `dominate' one. The cost was minimal and the setup was moved to several other vehicles over about 6 - 8 years as it worked quite well. (We actually mounted the transformer on the back of the speaker and had the "L-Pad" mounted through the lip of the dash out of sight with a small discrete knob for adjusting. The control heads for the radios were mounted just under the front edge of the front seat and the mics were hung right next to them rendering them almost invisible unless one actually really took the time to look for them. The antennas were thin wires running, centered, from the bottom to top of the front and rear windows in what I think was called a "T-Slot" configuration. He was able to get out just as well a a regular patrol car with only slight directionality! According to the owner of the shop he had used this type of setup many times on many different agencies' radios over the years with very few problems. I, even after doing several more such installations, was always amazed at how well such an installation could work. The speaker part, a fairly neat and straightforward setup, was fairly understandable. It was that danged antenna configuration that always puzzled me.
Anyway... If you don't want a lot of extra `electronics' and want to just go cheap and simple the way we did it with the audio transformer and "L-Pad" would probably be a good way to go. If you are going to already be putting in an external speaker you can probably go ahead and mount the transformer and control in that case and just run a second wire out for the second radio.
Just an `Olde Fart's' 2¢ worth. {GRIN!}
