I recently discovered that if you put a town portal scroll in your pet's inventory the pet will return instantly from the trip. This is great, but so far I've only managed to give entire stacks of the scroll to my pet, who will then gladly sell all of it, leaving me with no scrolls in case of emergency. How do I split a stack of items?
You can't manually split a stack of items unfortunately. The game will always try to stack all your items as you pick them up into a single stack if possible. If what you're picking up will not fit in the current stack or you don't have any of that item, it will create a new stack.
You can "cheat" this a bit. First find the item you want to put into a new stack. Before picking it up, give the rest of what you have in the existing stack to your pet and then pick it up (or go the other way around and have your pet pick it up). As long as you don't take it back from the pet or give up what you have, your stacks will be split.
You could also just drop the stack as well to achieve the same thing. But once merged, you're stuck with that stack until you sell it or fill it up.
I know, it's a pain, but that just the way it is.