Want to install 2 water heaters. How far apart can they be if they are installed in series. This refers to the piping for the water not electric

They can be right next to each other. Hot out and back into cold. For ease of possible service, I would separate by 2-3 ft.

Yup! No real minimum space requirements here, but like Brian said...separating by a foot or so is never a bad idea!

Mark