Are the items in a specific order? Do some items exist in one file and not the other? Why not have both sheets in the same workbook?

At any rate, this should present little problem. You can get the data from one sheet or the other quite easily using various means including sumif, lookup functions, etc. Heck, oif the item lists are identical, you don't need anything fancy at all, just sheet1!a1 +sheet2!a2. That though is risky so I would use something more definite.

Some sample sheets would help if this doesn't get you what you want.