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