SELECT `YEAR`, FORMAT(SUM(`count`), 0) AS `Total`
FROM (
SELECT `Table1`.`YEAR`, COUNT(*) AS `count`
WHERE `Table1`.`variable` = 'Y'
GROUP BY `Table1`.`YEAR`
UNION ALL
SELECT `Table2`.`YEAR`, COUNT(*) AS `count`
WHERE `Table2`.`variable` = 'Y'
GROUP BY `Table2`.`YEAR`
) AS `union`
GROUP BY `YEAR`