Suma de grupo de dos tablas según la fecha en MySQL

Esto no obtiene fechas con ceros, para eso necesitará una tabla de fechas o un procedimiento almacenado para recorrer las fechas. Una forma de hacerlo es una subselección de una consulta de unión (no probada):

SELECT commission_date, SUM(click_commission), SUM(lead_commission), SUM(total_commission)
FROM (SELECT DATE(click.time) AS commission_date, click.commission AS click_commission,
             0 AS lead_commission, click.commission AS total_commission
      FROM click
      UNION ALL
      SELECT DATE(lead.time) AS commission_date, 0 AS click_commission,
             lead.commission AS lead_commission, lead.commission AS total_commission
      FROM lead) AS foo
GROUP BY commission_date
ORDER BY commission_date