I live in a rural area in northern ontario and only have 100amp electrical service, in order to install a big enough electric tankless hot water heater I would need 200 amp service. So I am wondering if I could hook two lesser water heaters that require 100 amp service to work together to achieve the same result. Specifically the Stiebel Eltron tempra 29 that requires 3 50amp breakers vs. 2 Stiebel Eltron Tempra 15's which require 2 40amp breakers each. And would I need some kind of flow regulators or diverter's or could I just split the main line to feed both heater's and plumb the outflow into a collector.
http://www.gotankless.com/point-of-use-water-heater.html

You should check with the manufacturer of the water heater.

I will move this over to Plumbing see if anyone there can help.

Electrically, most of your electric service capacity will be just for these heaters.