I have been on the waiting list with the council for a three bedroom house for 15 year, I am in a 2 bedroom house with twins boy and girl who will be 16 on August what is the legal age for twins sharing a bedroom

There is NO legal limits on sharing ages.

There is no legal age for twins, or other siblings sharing, but around ten is generally considered to be too old. If you deliberately put twins of older than ten in the same room, it's likely that social services would be very unhappy and would question your fitness as a parent.

For now, I would split them up, and take whichever one is the same sex as you to share in your room.

There is no laws, anywhere, stipulating when a sibling can share a room.
When they become teenagers, children often need privacy from boys/girls, and only girls share rooms or boys share rooms, but girls and boys can still share rooms. Especially with twins. It can be detrimental to forcefully separate them.