Your parallel installation is wrong. The heater on the left is doing all the work. The piping that enters each heater must be exact in every fitting and length, as well as the hot water piping. Both must be exactly the same in length and fittings. If one pie is longer on one heater verses the other the water will take the shorter pipe (path of least resistance).
Here is a good PDF showing the proper way to plumb in duel heaters in parallel with a return line. http://www.hotwater.com/lit/wiring/315268-000.pdf