You have a many to many relationship, so you should use a join table that links the two line item records. This requires a serial number field that uniquely identifies each line item record.
ChangeTable::Item1ID = LineItems::ItemID
ChangeTable::Item2ID = LIneItems 2::ItemID
Where LineItems and LineITems 2 are separate table occurrences of the same data source table.
Now you can define calculations in the change table that refer to the data in each of the two line items.
Aha, yes, thanks. Wait, how would I populate the Change table?
Edit: nevermind, figured it out.