Use JOIN to join the subqueries and use ON to say where the rows from each subquery must match:

SELECT T1.col_a, T1.col_b, T2.col_c
FROM (SELECT col_a, col_b, ...etc...) AS T1
JOIN (SELECT col_a, col_c, ...etc...) AS T2
ON T1.col_a = T2.col_a

If there are some values of col_a that are in T1 but not in T2, you can use a LEFT OUTER JOIN instead.