You can use a Union query to do this:
SELECT tblA.Field1, tblA.Field2
WHERE tblA.Field1 <Some Criteria>
UNION ALL SELECT tblB.Field1, tblB.Field2
WHERE tblB.Field1 <Some Criteria>;
In a Union query the total fields must be equal from each table (as above two fields requested) there are some variations in data types that can be combined. For more detailed info see "Combine data in fields from two or more tables using a union query" in Access help.