I would not try to warm a room up with two 3KW fan heaters plugged into one twin socket.Yes, exactly.
Can you imagine the designer of the first 'double' socket being told that it only need cope with, say, 20A - 13A one side and 7A the other but also the other way round?
To daft to contemplate.
Actually I would shy away from running them on the same circuit usually or the same house if possible, you do not see as many of them as you used to do though but they are about.

