This can be done.
If I understand, you have two inventory lists which I presume are in two separate tables. You wish to create one value list out of these two tables. This requires a third table, a form of link table. It will contain five fields, one link field for each inventory table, one cover name field for each inventory table and one calculation field. The value list will be based on the calculation field.
Create your relationships so that your inventory tables link to the value list table via their primary key fields. In the layouts of the inventory tables, place the cover name field corresponding to the correct inventory table. When you create inventory records, make sure this is the only place cover names can be entered.
The last piece of the puzzle: the calculation field. Because each inventory record will create its own related link table record, only one of the two cover name fields will ever be populated. Make sure the result of the calculation field returns the name of the non-blank field.
Your value list will now display all cover name values.
I understand this solution may seem really complicated. If this was the case, feel free to contact me by telephone for more help.
I have sent you a private message (top of this page - right side - X Messages).