If you're bargain-hunting for fixtures and DIYing things, I get it. There are things in my house I'd never have if I'd had to pay someone else for a soup-to-nuts install job. That said, what you want to do is hugely expensive to put in and wasteful in operation. I stand by my guesstimate that you're spending $6k on water heating equipment for your uber shower. If you put in 2, each tankless unit (installed by a pro) will cost around $2k. That's including piping them up and running the stainless exhaust pipe outside. Firing those 2 burners at 180k to 200k BTU/hr likely will require upgrading your gas line, which could also easily be $1.5k plus. If you put in 2 showerheads and 8 sprays, that's around 25gpm of flow you need to provide. If the water piping in the home isn't big enough to provide that much flow, adding a booster pump ($600 to $800 installed) won't help. If it can't pull the flow required from the suction side, it will cavitate and you'll just destroy your $300 pump in short order. You'll need to have enough supply based on code for potential other simultaneous uses. Also, what's going to turn it on/off? It can't just sit there running when there's no flow downstream (i.e. nowhere to pump to). Thus the need to hire a professional if you really want to do this. You will also need to make sure that your drain piping can handle 25gpm. Again, there's a code calculation method to determine this. In operation, shower plus 2 sprays is ~7.5 gpm. That costs ballpark of $3/hr for gas (37F delta T, 90% efficient, $1/therm). It goes up to $10/hr to run flat out at 25 gpm (and I'm not sure even 2 tankless heaters put out that much heat). That's the quick-and-dirty, likely mistake-laden assessment of some of the issues of what you want to do. In more places than you'd think, water is a scarce, expensive resource that we don't really pay the real costs of producing. If everyone and their brother pulled out their flow restrictors or put in something like this, we'd be in big trouble.

There is a LOT of disinformation on these forums about on-demand tankless units, ESPECIALLY the electric units. First, energy cost savings is NOT a reason in itself to make the switch. No reason alone is sufficient. It is the total package, in light of your particular lifestyle, that will dictate the pro-con outcome of the decision making process. First, tankless units save space -- a lot of it, especially electric units that do not need to be vented. In Texas we have no basements, so that can be an important issue. Second, an electric unit is unlikely to blow up, unlike anything using natural gas. Third, there is an endless supply of hot water, which in a successive-morning-shower household like mine, is a really important issue. And finally, yes there is some documented comp fuel savings (electric tankless over electric tanks, or gas over gas), that is really offset by the added initial cost of the unit plus upgraded electrical service or gas supply line sizing. Moreover, there ARE full-house electric units out there that provide plenty of water, unless you plan on taking two showers simultaneously while also doing your laundry and running the dishwasher! And if that's your bag, you can run 2 or more electicals together. Or get a really big gas unit. Bottom line, nearly every nay-sayer of tankless units ignore the pros and focus on a single con - cost, cost, cost. Well, we all don't drive a Ford Focus for a reason. Some of us enjoy our Buicks, Porsches or Camrys even though another car would get us from point A to point B cheaper while using less fuel etc etc etc. Full disclosure: I have a Steibel Eltron Tempra 36 whole-house electric unit. Love it. My electric bill went up nary at all.

@Karen Williams This is the control panel on my tankless water heater. Water Set Point ... the output temperature the unit feeds into the hot faucets ... can be set in 1°F increments between 50°F and 140°F. Temp Up and Temp Down buttons adjust the Water Set Point temperature. Then press Temp Set to make it the normal at-all-times temperature if desired ... otherwise the adjustment is temporary and will revert back after a number of minutes. I have 103°F as the "normal" temperature and raise it when needed for washing dishes or clothes, without pressing Temp Set, and it reverts back to 103°F after 30 minutes so I don't have to remember to do that. That's just how my unit works, they're not all the same. Setting the temperature "at the faucet" means I set the water heater to the temperature of heated water I want/need "at the faucet" ... such as 103°F for a shower, instead of 120°F or 135°F which is much hotter than needed for a shower ... and I turn on only the hot faucet (or set a single-lever faucet to full-hot with no cold mixed-in). Thus, the tankless unit has a high-enough flow rate passing through it for the duration of the shower to insure it doesn't shut off due to too-low flow. 120°F, 135°F, 140°F output temperature requires mixing in cold water and turning down the hot flow at the faucet to get a comfortable shower, which reduces the flow through tankless possibly below the required rate which can cause it to shut off completely and the shower goes cold. Remote control panel means exactly that ... some tankless water heaters can have an optional extra temperature control panel installed at a convenient location such as bathroom or kitchen so the homeowner doesn't have to go down to a basement or out to a garage or wherever the heater is located to adjust the temperature. My heater doesn't have a remote control but it's located in a broom closet in my laundry room behind the kitchen near the center of the house so is easy to access any time I want to change the setting.

First off let me say that is an insane amount of water, for a residential setting. Really insane for California where there is a shortage of the resource to begin with. It is an interesting idea though. I know nothing about this kind of setup. There are only two people in my home and we get along fine with 40 Gallons of water in a tank with plenty to spare. When the kids were here though we could see issues if everyone decided they wanted to shower at the same time as we do have 3 bathrooms. If I were going to attack this idea, I would put the tankless after tank. In this way (at least in theory) the tankless would only operate at full capacity once the water in the tank was exhausted? Or as the water began to cool the tankless would ramp up. That theory based on how hot the water is flowing into the tankless and whether or not the tankless would operate at a lower setting or not at all if hot water was already flowing through it. I don't know the answer to either one of those questions. As far as those rain shower heads, not a fan. I have a family member who has one, not the best shower experience I have had. To each his/her own though.