A simple UNION ALL should make it possible.
SELECT date1 date, SUM(a) count_a, SUM(b) count_b
FROM (
SELECT DATE_TRUNC('day', date1) date1, 1 a, 0 b FROM table_a
UNION ALL
SELECT DATE_TRUNC('day', date1) date1, 0 a, 1 b FROM table_b
) z
GROUP BY date;
EDIT: Here's an SQLfiddle.