SELECT row_id, first_name, last_name FROM A WHERE row_id=the_rownumber
UNION ALL
SELECT row_id, first_name, last_name FROM B WHERE row_id=the_rownumber
the_rownumber should be the desired row number. Since you said there's no duplicate between the two tables, I just used UNION ALL. Otherwise you can use UNION DISTINCT to filter out duplicates.