Water – New T&P valves on water heater leaking, pressure driven from 65 to 165psi

a family I know moved into a house that's three levels, garage is at ground level and has two gas water heaters, one for each flat above. Both are 48 gallons. Each has four settings: A B C and Very Hot. Both are set at A. Both tanks have brand new T&P valves.

They continue to drip. So I tested the pressure and generally get a reading of 60 to 65 psi (when a water line is open, ~40psi) I turned up tank #1 one click (B) and by the time it finished heating the pressure rose to 165, the valve started dripping at 155. I turn that tank back to A and turned tank #2 up one click. This time the pressure only went up to 161 (only!) And tank number one started dripping at 155 psi. and tank number two started dripping at 160. I didn't even want to try what would happen if I turned both heaters on at the same time.

So here are the questions:

Is that a normal pressure range or is there a problem with the thermostats allowing too much flux in the temperature? (I didn't have anything to check the temperature so I don't know what the high/ low in a given setting is)

Since they have a common cold line supply, they're only about 4 or 5 feet apart is it safe to use one expansion tank- getting one that is qualified to handle 100 gallons?

The tanks I've looked at seem to have a max of 150psi. Obviously won't work.

I get conflicting recommendations on how to position the expansion tank I would think that you would wanted pointing up so the water pushes up against the air. Maybe it doesn't matter? Does the length of the tube coming off the T going to the tank have any effect?

There is a pressure regulator at the shutoff valve. There is no expansion tank. I decreased the house pressure by about 15psi when I left.

So, just add expansion tank(s)? New t-stats?

thanks

Rudy

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A bit of history if you're bored. They had a plumber over cos there was water on the floor near the tanks. Since it was not leaking when the plumber got there he said he had no idea what it could be. Then they said the water pressure was low so he turned it up. THEN they called me, I told them it's 50% extra when I have to come in after someone else (I think this should be a standard rule). Stopped by the house, left two plastic cups with pebbles in the bottom under the discharge tubes, next morning, the floor was dry, water in cups. Changed out both TnP valves, still leaked.