Untrue. You do not have to unhook the jumper to charge. Most built in chargers are 3 bank 12V chargers allowing you to charge 3 12V batteries to include the two that are hooked together. A 24V charger would allow you to charge both batteries at the same time with only 2 wires hooked up to the two batteries (connected to the 24V output)
2 batteries hooked up in a series will still put out 12V each when only using the pos and neg from each battery which is how we connected James boat with 2 total batteries providing 24V to trolling motor and 12V to everything else. Drawback to that hookup is that your batteries will last shorter each trip and electronic interference on your fish finder.
It will also receive 12V charge when charged that way. What you cannot do is hook up a 12V charger on the 24v outputs (the pos and neg that do not have the jumper).