I have an already plumbed house with electric 30gal and 47gal hot heaters supplying different areas of the house. I'm finishing a master bath with a custom walk in shower that will demand more hot water than what the 47gal tank can deliver. The two heaters are located under the house about 20' apart with the smaller one only supplying the guest bath and the larger one supplying the whole house including the master bath.

How can I plumb this so that I can use both heaters for the master shower?? I read the "in series" and "parallel" discussions but with the heaters not close to each other what's my best option?

Please include drawings when necessary
Thanks