I think your heaters are in a loopback mode. My definition. Input and output on the same end. If you are going to series them then feed one at the bottom, out at the top then to the other heater at the bottom and the final output at the top of the 2nd one. Personally, I would parallel them. Split the feed to both bottoms and combine the two tops. Better flow and better heat transfer efficiency. In parallel you wouldn't be feeding water that's already warmed to the 2nd panel.
Al