Really great advice on this thread! Like others here, I have 2 fixed mounts and a HH. The FB Icom M422 is equipped with a CommandMic that is available at the lower helm. The lower helm Standard Horizon VHF came with the boat when I bought her and is starting to show its age with a weak receiver. I plan to replace it with an Icom 506 with AIS when the budget allows. I agree that having identical or similar radios can alleviate the 'finger confusion' that can occur when moving from radio to radio. Both fixed mount radios have their own dedicated high quality antenna...
Shakespeare Galaxy.
I'm also a big proponent of remote speakers. My SH always had a remote speaker and I installed one at the FB when I installed the Icom. Since I seldom drive from the FB anymore, I moved the FB speaker down to the lower helm. I quickly learned that the CommandMic can control the volume at its own built-in speaker, but not the main radio or remote speaker. I needed to install a volume switch at the lower helm to control the remote speaker volume. Now that it's installed, all is well. The clarity of the received transmissions is so much better that I would never go back to the radio speaker.
I use my HH as a 3rd radio when cruising with friends. It monitors the VTS channel while the other VHFs are used for friend coordination and CH16 monitoring. If I'll be speaking to VTS, I normally switch them to one of the VHFs for better receiver clarity and transmitter power.
My radio antennae are separated by the width of my FB, roughly 9-10 ft. When I transmit on one radio, the other radio can pick up the transmitted signal on a nearby channel due to antenna proximity. The greater the frequency separation, the less of a problem. This is why it's important to keep the antennae well spaced. If you plan to use one antenna with two radios, you have a redundant system that is vulnerable to a single point of failure.
The same can be said for an electrical failure. I have both radios wired to the same bus for power, so I'm vulnerable there, but I have the HH for backup. I operate in the relatively protected waters of the SF Bay and CA Delta, so radio range is typically not an issue. If it was, I'd consider an external antenna connection for the HH radio.
I agree with what others have said about scan features. When the frequencies get busy, the scan causes dropped or missed calls that might be intended for me. So I find myself regularly switching the feature on and off as the frequency congestion varies.