You're good with what you're using UNION for, but you have to keep in mind that UNION vs. UNION ALL is the equivalent of using a DISTINCT or not on the query. It's not grouping/aggregating.
So, for starters, you want to use UNION ALL there because you might otherwise be losing data that you need to feed into your aggregation, unless you can straight dupe rows between the two queries.
Next, what you'll want to do is aggregate the union. It'll basically look like this:
SELECT
SiteCode,
SUM( SomeStuff) AS SumOfStuff
FROM
(SELECT SiteCode, SomeStuff FROM table1
UNION ALL
SELECT SiteCode, SomeStuff From Table2
) AS drv
GROUP By
SiteCode
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