Don't know, but consider that a typical WH can only dispense about 75-80% of it's contents before the temperature starts to drop significantly. Now, depending on the aquastat setting on the WH, you will be mixing at least some, and maybe a lot of cold water to make it comfortable. The first hour rating is more relevant when taking a shower, since the flow is lower. To fill a tub, depending on the supply lines, you'll be using water as fast as the pipes can flow it, and with a 3/4" supply, maybe as much as 18gpm. You might overrun either WH, as they wouldn't have much on-time to recover at a high flow rate. The way to maximize the hot water quantity, is to run the WH hot, and install a tempering valve to prevent that excessively hot water from hurting anyone. This way, you can get the equivalent of more hot water volume in the same tank since it is mixed with some cold on its output. This will increase your standby losses, though, and therefore decrease the overall efficiency. How much of the hot is in the percentage used will depend on how cold your cold water is...in the winter, you'll need more hot since the cold will be much colder; in the summer, the cold may be quite warm, and to cool the hot off, you'd need much more cold. So, it could be fine in the summer, and not enough in the winter.