Using the sidechain for auto-ducking, mic over music broadcast setting. Do you have to split the mic signal and go into both channels' sidechains or just one?
Each channel has it's own sidechain input, unless you are using two sidechain paths one would be fine. The reason why you may want to use both is apply it to two different sources/inputs. Or you may want to use one channel for side chaining, and the second channel regular input for something else.