I am trying to use my series connected 63 gallon high recovery unit and 50 gallon "normal" heater. Both natural gas, natural draft. Each seem to work fine seperately. When valved in in series though, the hot water in the morning only seems to be the temp of the first tank, which is 90. I have the second set at 120. Any suggestions. I have them valved so I can run each individually one at a time or either being lead or lag.