The most common cause of weeping from the TP valve is excessive pressure. The valve would "pop" full open at 150 PSI, but they are prone to weep beginning around 110 PSI.
Two things can cause pressure problem:
(1) The pressure regulator on the house main is defective. Commonly, pressure would be normal or low when any faucet is in use, then creep up past the set point when the water is off. House pressure will ultimately see full street pressure. You can get a $10 gauge at the hardware store with a telltale hand, to test this.
(2) Thermal expansion. Water expands when heated...not much, but enough! Some codes now REQUIRE an expansion o be installed, and all water heater manufacturers recommend it.
Since you have 2 WH, and the issue is especially noticeable when heating a lot of water.....I suspect you need the expansion tank. Again, the little gauge will help you evaluate .