Would it maybe be easier to create a separate table with Each Item and its location and the number in stock then create a portal on the first layout so when that field is modified on one record it changes on all others for that item name?
Separate tables would not be a good idea. But you can set up a self join relationship that matches to all records with the same value in the item ID and location fields. Then a calculation field can use the Sum or count function to give you the total on hand in that location.
A self join uses duplicate table occurrence boxes, Open manage|Database|relationships, select the box for your inventory table and click the duplicate button (two green plus signs) to make a duplicate table occurrence. You can then drag from the Item ID and location fields in one table occurrence box to the same field in the other.