How to join two tables with same primary key name but different values

You can do exactly what you requested with a full outer join:

select room_id, room_name, b.bedcount, k.chaircount
from   bedroom b full outer join kitchen k using (room_id, room_name)
;

This is almost equivalent to the query you attempted - but you need a natural FULL OUTER join rather than the (inner) natural join you tried. Note, however, that many (most?) practitioners view the natural join syntax with suspicion, for various reasons; the using clause as I demonstrated above seems to be accepted more easily. (Of course, even with a natural join, you might be best served still naming specifically the columns you want in the output.)

Although the more common approach for cases like this is a straightforward union all:

select room_id, room_name, bedcount, cast (null as number) as chaircount
from   bedroom
UNION ALL
select room_id, room_name, null    , chaircount
from   kitchen
;