When you need to merge two tables, you are joining them -

SELECT staff, 
       region,
       SUM(IF(YEAR(saledate) = '2020',amount,0) AS Total_20,
       SUM(IF(YEAR(saledate) = '2021',amount,0) AS Total_21,
  FROM (SELECT staff, region, amount, saledate
          FROM sales20
         UNION ALL
        SELECT staff, region, amount, saledate
          FROM sales21)
 GROUP BY staff, region;

SELECT sales20.staff,
       sales20.region,
       SUM(sales20.amount) AS Total_20,
       SUM(sales21.amount) AS Total_21
FROM sales20
  JOIN sales21
    ON sales20.staff = sales21.staff
   AND sales20.region = sales21.region
GROUP BY staff,
         region

You need to join 2 tables based on person and region for a one to one join. If you join based only on employees then it will do a one to many join so you will get corrupted output.

Another approach is to merge the data from the two tables and then perform aggregation on this basis. This should give you accurate results.

WITH combined_data AS
(
  SELECT staff,
         region,
         saledate,
         amount
  FROM sales20
  UNION ALL
  SELECT staff,
         region,
         saledate,
         amount
  FROM sales21
)
SELECT staff,
       region,
       SUM(CASE WHEN year(saledate) = 2020 THEN amount ELSE 0 END) AS Total_20,
       SUM(CASE WHEN year(saledate) = 2021 THEN amount ELSE 0 END) AS Total_21
FROM combined_data
GROUP BY staff,
         region