Water Heaters - Two Gas Heaters and Luke Warm Water

I've got a strange one. I have 2 gas water heaters where one feed the other. THe 1st shower in the morning is luke warm, but the temp gets very hot after about 5 minutes of flow. I'm guessing the temp of the 1st tank inline is hotter than the downstream tank that feeds the pipes. That's strange though, because I have the thermostat set higher on the downstream tank than the 1st tank. I turned on the tub and watched the burners. The 1st tank burner came on first since it was receiving input cold water. The downstream tank burner didn't come on until the water was amost cold coming out at the tub. The downstream tank burner went out first, too. I checked the temp at the drain on the tank with my hand, and it's just luke warm. I'm thinking bad termostat on the downstream tank. Any Guidance?