I want to mount my GPS in my dash and need to move the speedo and one tach. Right now there is one tach on either side of the speedo. Is there any reason that I could not mount the tachs side by side? Is it like keeping stereo speaker cable away from the power supply?

no reason. move em where you want em. no interference/magnetic flux/weird magic that will affect them.

Compass, however, is influenced by everything from your belly button ring to your 18" subwoofer.

(my fluxgate autopilot compass on the cruiser will float around 4 or 5 degrees when I crank the jams up high - if locked on a mag heading, the boat will also perform a little left-right dance too...)