I have a dual head shower in the bathroom. The shower is lukewarm all of the time. The valves that it uses has the hot coming in one side and the cold coming in the other. I bought an infared thermometer and with everything completely off the hot water is 132 degrees prior to the first showerhead. After the second shower head it is 109. And that is with the water completely off. I do have a whole home hot water recirc. It appears that the cold water is leaking into the hot water line. The tempurature out of the show is 80 degrees on full hot. If I turn the first showerhead on to full cold then it goes to 90 degrees. If I close the valves going to the showerhead then it warms up significantly.

My thought was to put some sort of check valves into the lines to be able to stop the flow of the hot into the cold or the cold into the hot.

The house is less than 10 years old.

Thanks for any help.

Are you using one mixing valve and one diverter to switch between the showerheads ?

I have a feeling that YOUR diagnosis is not correct and you need a professional to sort out the problem. Two common causes would be a misadjusted temperature limiter, or a bad check valve in the recirculation system.