Using a subquery in place of a SELECT fieldname ...or how to achieve the same result?
What you want to do is called a Cartesian product and uses CROSS JOIN:

SELECT NULL, t1.UID, t2.UID
FROM table1 t1
CROSS JOIN table2 t2
WHERE T1.`Groups` LIKE 'V'
    AND t2. `IsDefault` = 1;


This mixes each rows from table1 with each rows from table2.  (see sqlfiddle)