However, you could use one field as a "filter" on the second

thank you, i think that points me in the right direction. In my example I would like to put a field on items that says the name of the primary category

You might use the sourced from functionality then - Add a field to the item record, add a column to the transaction - have the column sourced from the item? (this might be an unnecessary step depending on what you are trying to do - saved searches would be able to pull in item fields in the search)