SQL Server 2017

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create table a (tstamp datetime2, weight int, destination varchar(100));
create table b (tstamp datetime2, weight int, destination varchar(100));

insert into a values
('2020-01-01 10:00:00', 40, 'germany'), ('2019-01-01 10:00:00', 50, 'germany'), 
('2020-04-01 10:00:00', 10, 'germany'), ('2019-04-01 10:00:00', 20, 'germany'), 
('2020-01-01 11:00:00', 40, 'congo'), ('2019-01-01 11:00:00', 50, 'congo'), 
('2020-04-01 11:00:00', 10, 'congo'), ('2019-04-01 12:00:00', 20, 'congo');

insert into b values
('2020-01-01 10:00:00', 40, 'germany'), ('2019-01-01 10:00:00', 50, 'germany'), 
('2020-04-01 11:00:00', 10, 'congo'), ('2019-04-01 11:00:00', 20, 'congo');

with combined as (
  select count(*) as counter, 'a' as source from a
  where destination = 'germany' and datepart(hh, tstamp) = 10
  
  union all
  
  select count(*) as counter, 'b' as source from b
  where destination = 'germany' and datepart(hh, tstamp) = 10
)
select
  sum(case when source = 'a' then counter else 0 end) as count_a,
  sum(case when source = 'b' then counter else 0 end) as count_b,
  sum(counter) as counter_a_b
from combined

count_a	count_b	counter_a_b
4	2	6