create table app_1(id number, sal number)
/

create table app_2 (id number, sal number)
/

begin
insert into app_1 values (1, 10);
insert into app_1 values (2, 10);
insert into app_1 values (3, 10);
insert into app_1 values (4, 10);
--
insert into app_2 values (3, 10);
insert into app_2 values (4, 10);
insert into app_2 values (5, 10);
insert into app_2 values (6, 10);
end;
/

O/P:

ID	SUM_VAL

1	10
2	10
3	20
4	20
5	10
6	10

SELECT app_1.ID, app_1.sal + NVL (app_2.sal, 0) sum_val
  FROM app_1 
  LEFT OUTER JOIN app_2 
    ON app_1.ID = app_2.ID
UNION
SELECT app_2.ID, NVL (app_1.sal, 0) + app_2.sal
  FROM app_1 
 RIGHT OUTER JOIN app_2 
    ON app_1.ID = app_2.ID

SELECT id, sum()
FROM (SELECT ... UNION ALL SELECT ...)
GROUP BY id