The attached test case works, giving the SQL:

select concrete2x0_.entityId as col_0_0_ from CON2 concrete2x0_ order by concrete2x0_.entityId ASC

select concrete1x0_.entityId as col_0_0_ from CON1 concrete1x0_ order by concrete1x0_.entityId ASC

and a warning: HHH000180: FirstResult/maxResults specified on polymorphic query; applying in memory!

Limits are not specified in the test case, regardless, this can be done in the database as:

(select entityId from CON2
union all
select entityId from CON1)
order by entityId ASC

And even with limits and sort conditions it can be done as:

select * from (
(select * from (select entityId from CON2 order by entityId ASC) where rownum <= 10)
UNION ALL
(select * from (select entityId from CON1 order by entityId ASC) where rownum <= 10)
) where rownum <= 10 order by entityId ASC

and also more generally with LIMIT and more sort columns, I believe.