Saw this today. 2 on demand water heaters hooked in parallel, not series. (meaning one or the other, not one to another) the idea here is, if the first one isnt cutting it the second one kick on. Problem is, the hot water will run, and when it switches, there will be up to 2 minutes of cold water. A little annoying when you are taking a shower. Looking at the plumbing, there seems to be no mixing valve. I wondered if they shouldnt be in SERIES, perhaps with a 5 degree temperature difference to prevent the second one from working too hard when it doesnt need to. What do you guys think?