As far as I know there are 2 one ways. Sell 1 of your items in the AH, not the whole stack (works with 11 or less) or just put parts of the stack into your Mog House.
To split a stack all you need to do is, Open your item menu, if your using PC hit the + key on the number pad to activate the sort menu. select Manual, highlight the stack you want to break and just keep selecting it. (Make sure you have the Auto sort function switched off)
__ This is indeed fast and correct way. But adding is, that i have the auto sort function on, but with this method (of getting 1 and 1 and 1) it doesnt sort it, so dont worry. Weird enough it doesnt sort that right away. When really wanting 2 stacks of 6, you should let the one with 6 being sold in your bazaar, which makes them gray and you cant do a thing with them. Go to your inventory and hit the sort button, and you will sort all the 1 pieces to a half stack. Go to bazaar again and switch off the other 6. NOW you do need to switch off the Auto Sort function or indeed this is gonna sort itself up again and you need to start over again.