Typically all you have to do (I tested this today) to split stacks, especially those supply packs, is to hold Left Shift and Drag and Drop it to another slot.
A popup will appear allowing you to choose how many you wish to split off.
As I said, I did this today from the Traveller's Chest in camp with the supply packs.
I saw this thread and figured I would relay the information.
Thanks.
I knew that Shift was supposed to split stacks, but I didn't know that you were supposed to drag a stack to a space in the same inventory.
