Have a licensed and insured plumber isolate these hot water tanks and pull the nipple from the outlet side of each tank.
ILL bet you a dozen donuts these nipples are close to 70% blocked with sediment.
This is a common problem when installers do not allow for the mixture of two types of metals electrolytic action occurs.
Sometimes the mineral deposits actually bake on the inside of the out let piping and this is why they should be inspected ALSO very important have the Licensed plumber not only test your T&P valves on each tank but possibly replace them.
The plumber should also use the blow down valve (drain cock) on each tank to flush out any sediment that has accumulated on the bottom of these tanks robbing you of efficiency.