The first thing that stands out is why do you have all the NOLOCK hints? You are aware of the problems that this hint causes?
Second you need to treat each SELECT in a UNION as its own grouping set
So you would group up the first select, union, group the second select
SELECT
Col1,
Col2,
FROM
Tab1
GROUP BY
Col1
UNION
SELECT
Col1,
Col2
FROM
Tab2
GROUP BY
Col1