How can this happen?
I own a double with tenants on each side. There is only one water meter so both are on the same system. But they each have a separate hot water heater. They both say that when the tenant on the OTHER side runs hot water while they are in the shower, their water gets cold. How can one tenant's hot water use affect the other's hot water use? I had new hot water heaters installed when I rehabbed the place 6 months ago.
You will need to supply a larger cold water line before you split to the two heaters.
You may only be sized for the use of one heater.