Single select on identical field names without collating conflict ?

OK gurus , listen up....

Have two tables A and B. Want to do a single SELECT if possible that does this

SELECT CASE WHEN A.Code IS NOT NULL THEN A.Code
       ELSE B.Code
GROUP BY A.Code,
         B.Code

This gives a collation error . Also tried COALESCE, also gives collation error.
I am having to do this along with another 20 fields and only suggestion so far is to do a subquery that essentially replicates the main query with the added pain
of giving aliases to several field names.
SQL is very poor !!!