HEY JOE! It's Mark, the guy who worked on your carb at Mud Lake a couple of years ago! Hope to make it to the Coastal Get Together this year.
Anyways, to answer your question, the guages share a common power lead. The sending units provide a fluctuating ground which changes the reading on the guage. So whenever more than one guage acts up at a time, it is much more likely to be a power issue than a ground issue.
I would identify which wire in the plug brings the power to those two guages and bypass it temporarily with a switched hot wire from the fuse block to see if the guages read normally.
Good luck.