Merge two QueryOvers

Question

If I have two separate QueryOvers; for instance:

var q1 = QueryOver.Of<Customer>(() => customerAlias)
    .select(x => x.RegionId);

var q2 = QueryOver.Of<Store>(() => storeAlias)
    .select(x => x.RegionId);

And I wanted to check, via a subquery, whether or not an item appears in either of these records, I had thought I could try something like:

query = query
    .Where(
        Restrictions.Disjunction()
            .Add(Subqueries.WhereProperty<Region>(x => x.Id).In(q1))
            .Add(Subqueries.WhereProperty<Region>(x => x.Id).In(q2))
    );

But when this section of code runs, the query doesn't seem to return the same type of results; where the code later tries to extract the ids, I get the error:

NHibernate.QueryException: 'could not resolve property: Id of: Models.Region'

From the code:

var allRegionIds = query.Select(x => x.Id).List<long>().ToList();

If I change the code back to what I had (below), which only checks in one subquery, then I don't get this error.

query = query
    .WithSubquery
    .WhereExists(q1);

Any thoughts on either:

1. How could I merge two QueryOvers together so I can use my working code that only checks against 1 QueryOver
2. Why am I getting the error further down in the code when I use the code which checks against 2 QueryOvers?

Any help would be greatly appreciated