Get rid of the two separate fields. Replace them with a join table to faciltate a many to many relationship. This is the most flexible way to set up a many to many, but also is likely to require more design changes on your part to implement:
YourTable::__pkYourTableID = YourTable_Inventory::_fkYourTableID
Inventory::__pkInventoryID = YourTable_Inventory::_fkInventoryID
Add a calculation field, cIDList, with this expression: List ( id1 ; id2 ) and select Text as the result type.
Then relate your tables like this:
YourTable::cIDList = Inventory::__pkInventoryID
That's much simpler to set up, but also can be a lot less flexible.