Nope, no damage at all. How could there be damage? They act as one when charging - the charging system sees one big battery to charge. The batteries themselves decide how much they'll take of the available current.so with this isnt there still damage being done? Even with parallel charging I always thought that unequal charge percentages of like batteries still reduced life expectancy.
You are better off charging both fully and resetting the IBS if you change just one of the batteries, and don't put a fully charged battery with a dead one or low charged battery but seeing all the stuff out there on the details and science, I'm seeing a lot of old-school thinking out there even on battery sites.