The easiest solution and probably the cheapest I can see is to have an electrician rewire the water heaters you have. Most electric water heaters have 2 elements but only 1 is powered at any given time. It would take adding an additional supply wire to each water heater so that each element works independently but would easily take care of your problem.
I'm sure that this can be done, but not as simple as it sounds.
I had a electric hot water heater several years ago and my memory is a little fuzzy. I believe that there's only one thermostat and one over temp cutout. in order to power both elements one would have to add both a second thermostat and an over temp cut out.
I think one would have to cut open the outer shell and "weld" some sort of mounting hardware for both devices.
Additionally, I'd suspect that one would be breaking codes somewhere, probably both plumbing and electrical codes.
The alteration of a appliance and using two power feeds to one device.