Well, this trigger is currently setup for combining the exact same types of items. If you want a full blown recipe system that say combines Boots of Speed + Gloves of Haste = Power Treads, then you'll need to expand upon the concepts found in this trigger.
But before doing that, you can always try a pre-existing system for your item recipes:
K GUI Recipe System, allows you to craft multiple items.
www.hiveworkshop.com
Also, here's a system I made for someone a few years ago:
EDIT: Solved, Uncle found the solution as usual :P I've been fiddling with this for about 6 hours before trying to post here, but sadly I'm not able to code this one out so I will need help with it. Basically, I have a shop. The shop contains items, and if you gather 2 items of the same kind...
www.hiveworkshop.com
It seems to have a working recipe system that utilizes a Hashtable, although I can't promise that it's flawless.