it is a moen assembly sorry I don't know the actual valve models--no paperwork on that from the original owners and the plumber took the info I used to order the part--the part I ordered/replaced was 1423 Balancing spool Moentrol maybe something like this for the two-head control http://www.moen.com/products/T4111ORB ours is oil-rubbed bronze and it is installed with the lever at the top and the pull-out valve under it which looks the opposite of the Moen site photo the info on the Moen site says that it should turn from 'full cold to full hot' but there are stops the plumber came back out and said that when he put in the balancing spool he had opened the hot stop all the way and that if he closed the cool stop that it would not do any good because the balancing spool would basically just reduce the flow of water to make sure the balance of hot/cold stayed the same... he said that there is movement underway to basically take the choice out of amount of hot water coming through faucets and showers--that safety of not burning/scalding people is overriding someone's desire to have a 'HOT' shower... when I asked about replacing the valve/lever with one that would swing right/left to allow me to regulate the balance of hot/cold... he said it would be too expensive to actually change the control to that type of fixture because we would have to open up the shower wall--take out tile--replace it... so we are basically stuck with either setting the water heater to higher temp because that will bring hotter water to the mix and will be expensive choice or just learning to live with it like it is I plan to take a shower in the two upstairs showers and see if their water is hotter than the one downstairs--- he said that the master shower is probably farther from the water heaters--and it might be farther than the other downstairs shower by just a little...which would influence the water temp at the head... both water heaters are upstairs and closer to those showers... it is just very frustrating to find out that something we really did not consider all that much would become so frustrating... I don't take long showers--and I don't use the big tub at all--don't consider that I waste water compared to larger families or people who take a 15 min shower...so this irks me to no end-- our old house with its old shower had wonderful hot water and I am really going to miss it... Here is a link that might be useful: shower valve

You actually want to run them in parallel. Right now, there's nothing to control where the hot water flow comes from at a given fixture, besides the piping losses (length/size of pipe, fittings). If you have a bigger line to/from the bigger heater, and it's closer to the cold water entrance to the house, hot water will want to come through it preferentially. You can get some brands (I know you can for Noritz) where they communicate between 2 heaters in parallel. But I believe that's really intended for where they're sitting next to each other. With them on the opposite sides of the house, you'll probably need to do what we did. We have a tankless on one end of the house for 2 bathrooms. The older gas tank heater is in the garage on the opposite corner of the house. In the crawlspace, there's a closed valve on the hot water line in between the 2 bathrooms and the rest of the house. Basically, this zones the hot water system. If either heater is out, I can open the valve and feed the whole house. But the 2 heaters don't fight each other under normal circumstances. To get your smaller heater to work, you need a closed valve somewhere. It might be on the extension/connection you mentioned in your OP. But that depends on how much stuff is on the 2 systems. I may be wrong, but I think zl700's check-valve solution would still lead to zero flow through one unit most of the time. That'd be the smaller one, since it sounds like it's running the least now.

I'd venture that the fasteners used to secure the shower enclosure to the top of the curb went through the waterproof membrane. That's hoping that there is a membrane wrapping the top of the curb to begin with. Regardless, it's likely that water is getting to the interior of the curb, saturating the curb structure, and the stain could be a mineral formation, or a bleeding through of some component used during construction.

I looked up the manual and Hybrid is the correct setting for it to use both the heat pump and the electric elements. I'd set the temp to 130. It has a first hour rating of 65 gallons - and if you aren't getting anything close to that, you have a problem with the water heater. Since it's not showing any errors - check to confirm that the "cold" is is plumbed to the cold water inlet. A broken dip tube could cause the same issue. Since it's always been "not quite right" I'd take a hard look at these two things. Then I would check the bottom element. Again - on hybrid... at the worst it should perform as a 50 gal electric water heater.