I need a third table C with one dropdown field from each table A and B plus one calculated field.
Lookup fields and calculated fields in tables are best avoided.
The evils of lookup fields - http://access.mvps.org/access/lookupfields.htm
About calculated table fields - http://allenbrowne.com/casu-14.html
While I'm at it, so are multi value fields
About Multi Value Fields - http://www.mendipdatasystems.co.uk/multivalued-fields/4594468763
Calculations should be done via forms and queries on the fly and not be saved as data.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.