Merging your two separate tables would seem the simplest solution here. Why have separate tables?
You could set up a separate table with one record for each Sales Order record as well as records for each Invoice record, but you'd need to script a process that creates a matching record in this table each time an Invoice or a Sales Order record is created. Then you could base your value list on this table--but that seems very cumbersome when compared to simply putting all Sales Order and Invoice Records in a single table. You can add a field to such a table to use to identify each record as one or the other so that you can work with just one type of record at a time...
Understand and thanks.