If the only problem is running out of hot water, just get a bigger tank heater.

That is the simple solution for a simple problem.

A tankless heater will provide endless hot water, but there are other variables that it introduces, which may cause undesirable side effects.

Each time the water is turned off, even for a fraction of a second, the tankless heater goes cold. This causes a "cold water sandwich" to come through the pipes. This can be a real annoyance, and even a safety hazard, because the temperature changes from hot to cold and then back to hot again so quickly, it can be a scalding hazard.

Reduced flow is another issue. Tankless heaters achieve the correct temperature by regulating the flow of water at the same time as regulating the flame from the burner. When it needs the water hotter, and the flame is already at the max, it reduces the flow through the heat exchanger to get the correct temperature.

Finally, there is the issue of getting enough gas to the unit for the burner to have full flame. 200,000 BTU's needs a big gas line, and it should really be the first draw on the line.

US DOE or Energy Star web sites have tools for determining lifetime costs for various water heating equipment. You might look it up. When i last checked, tankless was pretty costly. The initial costs are quite high.

The key is to find out how many gallons per minute of hot water you need to run both showers. Then you can either re-size your tank heater to meet that, or size new tankless heaters to match that.

Depending on the incoming water temp., more than one tankless unit may be needed to run both showers. Your plumber can easily solve those equations for you.

"The key is to find out how many gallons per minute of hot water you need to run both showers."

Gallons per minute is not the issue. The OP is not running two showers at the same time. They have one shower with body spray nozzles. The current heater has enough GPM to run the shower with body sprays just fine. The problem is that another person can not take a hot shower after the first person is finished. The issue here is total volume available, not gallons per minute.

I think jca1 was counting the shower and the side sprays as adding up to two showers. Either way, you got to crunch some numbers instead of trial and error*. Temp differential and GPM for everything you want to run at once in the case of tankless. For tank, GPM*time, temp differential and reserve/recovery.

*Analogous to measure twice, cut once!

Thanks. DH had a authorized dealer from Rinnai come out last night to give us a quote. He came a half hour early so I was not home. He is going to see if we need 2 or not.

According to him we have a 50 gallon tank now. I would love not having to worry about a huge water leak when it dies like both my parents and my mother in law have had in the past.

"I would love not having to worry about a huge water leak .."

??

Tankless water heaters can leak, and they have an endless supply of water with which to do so.

They're more expensive to buy and install, more expensive to maintain, and more expensive to operate. The only reason to buy one is if you need an amount of hot water that a large tank can't supply and you don't care how much it costs.

Otherwise, as others have said, you'd be better off with simply a bigger tank model.

"The key is to find out how many gallons per minute of hot water you need to run both showers."
Gallons per minute is not the issue. The OP is not running two showers at the same time. They have one shower with body spray nozzles. The current heater has enough GPM to run the shower with body sprays just fine. The problem is that another person can not take a hot shower after the first person is finished. The issue here is total volume available, not gallons per minute.

You're right, I misread. I thought the OP meant two showers could not be used at the same time.

A tankless probably won't be able to supply enough hot water for 6 body sprays, a shower head, and a handheld. Even a 100 gallon tank might not leave you with enough water for that second shower should the first person add a minute to that shower. The solution is to not use all of those sprays on an everyday basis. Save that for the weekend when you can have two people in that shower using all of the doodads at once. Now you don't need a second water heater or to spend 8K on installing two tankless.

each body spray is 1.5 rpm, a good shower head is 2.5 gpm, but I have seen them up to 25 gpm the handheld is the 1.5. 13 gpm is the number for everything depending upon the shower head. Most people who build these types of appliances want to use them for longer than 7-8 minutes at a time, which is the max your 100 gal tank would supply. It is stunning to me that these get built and folks do not understand that they need a lot of hot water. I can tell you you are not alone as I have seen plenty of this over the years. So many people have asked me why they were not told that these fixtures would require so much hot water. The answer is that if they knew up front they would not have sold the fixture. I represented Rinnai for 20 years so bias noted, but I also know how they work.

An RU-98i will deliver 5.5 gpm at a 70* temp rise. more with a lower rise. So from 50-120 you get 5.5 rpm. Your thermostatic shower valves are set by code at a max of 112*. Three units, two may suffice, will give you the water you need to deliver the comfort you hoped to enjoy when you built your shower. BTW, the RU-98 has a feature that IF years down the road the water heater springs a leak any water in the vestibule it will drive the cold water inlet valve closed to prevent water damage. Pretty neat.

You can certainly install enough storage to feed the shower, but I would ask how many minutes a day you are using this fixture. If you took a 30 minute shower and had the tankless you would have a very comfortable shower of whatever time you desire. At whatever your flow rate ends up being, how many gallons of storage do you need for a 30 min shower. If you install the multiple tanks you will maintain the temp of a few hundred gallons 24/7/365. Figure 8.33 lbs/gal plus the tank weight for structural purposes. If you think the tankless is expensive, try that bill on for size. The other issue is that with the 2-RU98i's once you step out of the shower it will be off and not operate until someone opens a tap.

I am sorry you are faced with this. Not putting enough capacity in and using only the shower head will drive you crazy every time you step into that shower. These types of fixtures are not cheap to install. That said, you built it. Now you have to feed it. If you have any specific questions, fire away.