How to merge two excel files?

If you want to merge PST files into a single file, you should try an easy manual trick. The manual trick doesn't require you any paid software. All you need to use is Microsoft Outlook client, that's it. It has a built-in Import feature which you can use here. By using that, you can import mailbox content from two or multiple PST files into a single file. Also, you can use drag & drop feature here. 

Refer to this blog post for complete information - http://www.softknoll.com/merge-pst/

Assuming for the sake of the example that you have your "productcode" data starting with the header in A2 and the "inventory" starting in B2 and the "testproduct" data in Row6 on both workbooks. Also, you named the data range on the second workbook "newdata." If you want the updated inventory info to appear alongside the existing data, in C6 on the first workbook enter: =VLOOKUP(A6,Test1A.xls!newdata,2) Test1A = the name of the second file. This will give you the updated inventory number entered on the second book (10), in this case named "Test1A," placed alongside the original inventory (99). Note: You must have unique data entries for this to work. This will list the update beside the original inventory, not overwrite it.