It seems that multiple-battery parallel-charging cables, such as that one, are working out ok for many folks.
But.........I thought --- correct me if I'm wrong, here -- that one of the considerations is that there can be problems unless all of the batteries intended to be charged are at approximately the same level of discharge.
And getting charge readings on UM batteries can be problematic.
Rather than get into that tall grass, I've stayed with charging only one battery per charger.
Is there a definitive answer to the question of whether my concern is unfounded?