so, do I have any concerns with the two marine band radios but need to only monitor the 2-meter and shut down the marine receivers when TX on 146. I can get better than a 1/2 wave separation between antennas.
The marine radios will probably shut themselves down when you transmit on 2m due to the strong signal.
Same with the 2m radio when you transmit on the marine band. It just isn't practical to install good
enough filtering to prevent it, though you may find some paging filters that will reduce the strength
of the marine signals arriving at the 2m rig.
When I was with Search and Rescue we had about 9 antennas installed on the roof of our van within a
6' square area: marine, aircraft, VHF high band and low band, UHF, CB, scanner, VHF telephone, etc.
All the radios suffered desense, but none of them were damaged: we just knew that we couldn't hear
on one when transmitting on another.
Stacking the antennas vertically will help, and you may get some interactions that increase the SWR
if they are too close side-to-side, but usually it is a matter of "try it and see".
I would be concerned, however, about your plan to put the 5/8 wave antennas on elevated mounts.
They really are designed to be mounted directly on a flat surface, and the RF performance may suffer
as they are raised above it. (This is especially true of using sloping radials below a 5/8 wave antenna -
it raises the angle of maximum radiation above the horizon and decreases your effective range.)
I think you will be better of with antennas that don't rely on being mounted to a flat sheet of