Roughly you can figure that 1/2 of the water heater's capacity is available to you before it becomes diluted enough with cold water to cause a temperature drop to your water at point of use. That means that you can have a 3 minute shower before it starts to go cold. Even 2 100 gallon heaters would only give you a 7 minute shower. Plus, you're going to have to up your drain size on your shower and a host of other issues, such as not having enough water pressure to use that much water at once.

You're talking a commercial installation with pressure tanks and commercial sized heaters and 3" drain lines if you install what you're planning. Not only is this mucho expensive to install, but it's mucho wasteful of our planet's limited resource: fresh water.

I believe its closer to 70%

50 x .7 = 35 x 2 = 70 / 13.3 = 5 minute shower

If you have ever faced 6 body sprays for 5 minutes, thats usually long enough, remember with that much water your eyes will be closed

Wouldn't it also depend on the water heater's temperature setting?

Thanks for the replys. I've never had body sprays so maybe 5 minutes is plenty. I plan to set it up where you can run 3 or 6 sprays at a time (or 4 and 2).

I am a little confused about the math. If you have 120°F temp water mixing with 45°F water (80% 120 + 20% 45 equals 105) then wouldn't the math be:
50x2/.8=125 / 13.3 = 9.4 minute shower?

The above calculations hold true if the water heaters are connected in tandem however, if two 50gal water heaters are connected in series you begin with 100gal of water and as the dilution begins in the first water heater, the slightly cooler water enters the second tank where is is raised to the desired working temp, thus you can create a nearly continuous flow.

In addition, when connected in series the first tanks temp control can be set about 10deg higher than the second tank thus under lighter loads the water will pass through the second tank without triggering the burner. In this manner under lighter loads the second tank functions solely as a storage vessel thus saving energy.

On the other hand, there is a much bigger problem that generally goes unnoticed by homeowners. While most homeowners think of a shower in terms of the entire shower enclosure which is primarily intended to be a single occupant facility code must be capable of defining both the supply and drainage requirements of all showers, whether it is the single occupant residential facility or a large gang shower such as might be found in a school or factory locker room, thus for the purposes of estimating supply and drainage the code defines a shower as the shower head, thus when two or more shower heads can be operated simultaneously, we must allow the full code minimum water supply for each shower head. By example, under the IRC even though shower heads are restricted to a maximum flow rate of 2.5gpm, code requires that we size the piping for 3gpm. If we then design a shower facility with one shower head and 6 body sprays code would require the supply piping to be sufficient to supply 7 showers at 3gpm per shower for a combined total of 7x3gpm =21gpm.

When using copper pipe code limitÂs the velocity of flow to a maximum of 8ft/sec thus to get the code minimum 21gpm supply, it would require an 1-¼" water line and 100gal water heaters.

In turn, we must also compute the drain load at 2dfuÂs for each shower head, thus the combined drainage load for this shower would be 14dfuÂs, which would require a 3" drain line.

I would strongly suggest you have a long talk with your local inspector before commencing this project.