In a UNION, you can only have 1 GROUP BY and 1 ORDER BY and each has to e after the last UNION SQL. You also have to reference the GROUP BY and ORDER BY items by number. How about something like:

select Field1, sum(Field2) from Table1;
     where Field3 <> "XXX";
union select Field1, sum(Field2) from Table2;
     where Field3 <> "XXX";
     and Field1 not in (select field1 from table1);
     group by 1