T1                                  T2
num    qty                          num    qty
 1        10                             1        5 
 2        20                             3        30 

 1      15 
 2      20 
 3      30 

 --1 вариант
 
declare @temp table
(
	num1 int null,
	num2 int null,
	qty int null
)

insert into @temp
select t1.num, t2.num, isnull(t1.qty,  0 )+ isnull(t2.qty,  0 ) 
from t1 full outer join t2 on t1.num = t2.num

select num1, qty from @temp where num1 is not null
union all
select num2, qty from @temp where num2 is not null and num1 is null


 --2 вариант
 
declare @temp2 table
(
	num int null,
	qty int null
)

insert into @temp2
	select * from t1

update t
set t.qty = t.qty + t2.qty
from @temp2 t, t2
where t.num = t2.num

insert into @temp2
	select num, qty from t2
	where num not in (select num from @temp2)

select * from @temp2