Children who have reached the age of ten must be separated in their beds

It is not permissible for two who have reached the age of
puberty to sleep together under one blanket. The Prophet (peace and
blessings of Allaah be upon him) said: “Teach your children to pray when
they are seven years old and smack them if they do not do so when they are
ten years old, and separate them in their beds.” Narrated by Ahmad, 6689;
Abu Dawood, 495; classed as saheeh by al-Albaani. 

If this is the ruling concerning those who are ten years old,
then how about those who are older than that, who should not be in one bed
but rather should be in two separate beds with two separate blankets? If
there are only two covers and one of them is not sufficient to ward off the
cold, then each person should take one cover and fold it in half so that it
will be thicker and will ward off the cold, or he should buy another
blanket. 

Al-Haafiz ibn Hajar said concerning a number of people sleeping in one bed: It was proven via another isnaad that this is subject to the condition that they do not sleep under one blanket.  Fath al-Baari, 7/204.