SQL> select sum(id) from t1;

   SUM(ID)
----------
         6

SQL> select sum(id) from t2;

   SUM(ID)
----------
        14

SQL> select a.s+b.s2 from (select sum(id) s from t1) a, (select sum(id) s2 from t2) b;

  A.S+B.S2
----------
        20

SQL>