What happens if I have two water heaters and one's over-pressure relief valve fails?
An over-pressure in one water heater tank should be relieved by a pressure relief valve on the other tank, IF both tanks are fed by the same cold water supply and there is no check valve between them.

However, this would not be a acceptable situation because the valve on a tank is a temperature or pressure relief valve. If there was a runaway on one tank and the relief valve had failed closed, this would lead to water at a temperature at or above the normal boiling point, then this scalding water would enter the hot water supply system. It would be injurious to users and could damage PEX hot water supply lines which are not intended to be in contact with pressurized water at 212 F (100 C).