Most of the time I'd guess the difference between the batteries would be <1V by the time I plugged them back in. On the other hand, it's not something I want to be worried about monitoring all of the time.

I found this controller that has dual outputs for charging 2 batteries independently. Might be worth it to swap out for this (and then hold on to my current controller for when I inevitably want to add a 3rd battery to the setup 😂)