How do you hook up two water heaters in line?

you want your cold line coming tying into both inlets on the water heaters and the hot lines tied onto the outlet side of the water heaters evenly (so they get even draw and one water heater doesen't have to work harder tnan the other). your recirc line coming in ties into the cold line in beetween the two water heaters with a circ pump going twards the cold line and the other end of the circ line tying into the hot line on the furthest fixture. on the cold line coming in you need a ball valve, check valve and expansion tank before it ties into any of the heaters.

NOTE: The solution above is for tanks in parallel not in series.