How can i make 2 items stack?

How can i make 2 items stack?

I need to know how to make 2 items stack

exampleotions.I want to make 2 healing potions stack so when i buy 2 they will be with 2 charges

i tried to make it by trigger but i screwed up!

help a lil bit?

Here a link

http://www.wc3sear.ch/viewtopic.php?t=15018&highlight=

My code at the bottom. it's cleaner and better