Table A                         Table B
  date     | name | amount        date     | name | amount
-----------+------+--------     -----------+------+--------
2018-01-16 |  A   | 2  <==      2018-01-16 |  C   | 1          tuesday
2018-01-16 |  B   | 3           2018-01-16 |  A   | 2  <==
2018-01-16 |  A   | 4  <==
-----------+------+--------     -----------+------+--------
                                2018-01-11 |  A   | 8  <==     thursday
-----------+------+--------     -----------+------+--------
2018-01-02 |  A   | 2  <==                                     tuesday
2018-01-02 |  A   | 10 <==
-----------+------+--------     -----------+------+--------
2017-12-25 |  C   | 4           2017-12-25 |  A   | 2  <==     monday
-----------+------+--------     -----------+------+--------
                                2017-12-26 |  A   | 7  <==     tuesday
-----------+------+--------     -----------+------+--------
2017-12-07 |  A   |  9 <==                                     thursday

SELECT
    t.`day` AS `day`,
    SUM(t.`amount`) AS `Atotal`
FROM (
        (
        SELECT
            WEEKDAY(`date`) AS `day`,
            `amount` FROM `tableA`
            WHERE `name`="A"
            AND `date` >= (DATE("2018-01-16") - INTERVAL 6 DAY)
        )
        UNION ALL
        (
        SELECT
            WEEKDAY(`date`) AS `day`,
            `amount` FROM `tableB`
            WHERE `name`="A"
            AND `date` >= (DATE("2018-01-16") - INTERVAL 6 DAY)
        )
    ) t
GROUP BY t.`day`

day | Atotal
----+------
 1  |  8      (tuesday: 2018-01-16, 2 + 4 + 2)
 3  |  8      (thursday: 2018-01-11, 8)

day | Dtot | Atotal
----+------+------
 0  |  1   |   2   (2017-12-25) | (2)
 1  |  3   |  27   (2018-01-16, 2018-01-02, 2017-12-26) | (2+4+2+2+10+7)
 3  |  2   |  17   (2018-01-11, 2017-12-7) | (8+9)

SELECT
r.`q_day` AS `day`,
COUNT(r.`q_day`) AS `Dtot`

FROM(
SELECT
    WEEKDAY(q.`p_day`) AS `q_day`
    FROM
    (
        SELECT DISTINCT `p_day`
        FROM (
            (SELECT DISTINCT `date` AS `p_day` FROM `tableA`)
            UNION
            (SELECT DISTINCT `date` AS `p_day` FROM `tableB`)
        ) AS p
    ) AS q
) AS r
GROUP BY r.q_day