We want to validate values of the dimension [Job No.] from Query 1 against values of [Job No.] from query Valid within Webi report.

We can create a detail object [Valid Job Name] for merged object [Job No.] that points to unmerged dimension from the query Valid [Valid].[Job Name] and use it in the following expression to check if the value is valid:

=Not(IsNull([Valid Job Name]))

So the point is to use unmerged object expression like =Not(IsNull([Valid].[Job No.])) will not work.