Hello: Ideally, water will never have the opportunity to sit and stagnate in a tank. With parallel plumbing, you likely would need to drain the unused tank (and of course turn off power to it). With series plumbing, you would heat with the downstream tank. Flow could be reverse with valving, but it would be complex.
I’ll guess from the photo that you have parallel plumbing. One can “temperature balance” tanks in a low tech way. To do this, plan on taking a big bath! Let water run until you feel cooler water coming from one tank before the other tank. (use the same hand to test temps) Then slightly reduce flow to the tank that had cooled first. Ideally, you want both heaters to go cold at the same time. An infrared thermometer can do this nicely if you happen to have one. 😎 Note that over time things change. Rust can build up in one line, throwing balance out the window, so periodic re-balancing is needed with parallel systems.