[chumbucket]

Yeah, what you're wanting to do is a little complicated but I would think that the charger leads could be run through a switch to change from one set of batts to the other. There is a potential problem with the common negative posts, but that's a concern whether charging or not. When multiple batteries are tied in together, the two are only as good as the weakest battery. It's important to maintain them equally because the weaker of the two will drain the good batt.