Try running about about a 1/2 gallon of water into a bucket from the drain on the first heater and check the temp of the water.

Then run about 5 gallons of water from a hot water faucet in the house. Then draw another 1/2 gallon on water from drain at the bottom of the first tank and check the temp of the water again.

If the water temp is close to the same at both readings then the heaters are most likely hooked up backwards or the dip tube is damaged. If the second reading is a lot lower than the first then the first heater is most likely conected properly. This is assuming that both elements in the first heater are working properly.

To test the second heater the same way, the water in the first heater would need to be cold.

Hope this helps!