Your solution works if all you wanted to do was make sure either shower unit had a certain amount of hot water available. When its heater ran out of hot water, you will shower in cold water, but the other one will still have hot water. In a series installation, either shower would have TWICE the amount of hot water available, AND it would only create a problem for the second showerer if the first one were a glutton, in which case he deserves to get a cold shower the way you did do it. However, I might have connected BOTH of them to the hot water from the original heater.