My son and his wife are expecting twins in March. Do you have some guidelines or info sheets about cribs/cots and sharing sleep space (how to let them be near each other or together safely), particularly if bedroom space is limited? Also, is it safe to stack one cot mattress on top of another (matching mattresses) in a wooden cot to raise the height of the babies to save bending. Mum is 6ft tall. Thank you.

Red Nose has the following information about sleeping twins safely.

Research has shown that the safest way to sleep twins in the home is in their own safe sleeping place (cot, portable cot) in the parents’ room for the first 6-12 months…...........
If twins must share the same sleep space, a cot is the most suitable, do not use bedding (such as sheets and blankets) and place them at opposite ends of the cot.
An infant sleeping bag used for each baby may be useful in this situation.
When the babies have reached the age where they can move freely around the cot, they must be placed in separate cots.

See article for full information & graphics
https://rednose.org.au/article/co-bedding-twins

It is not advisable to stack one mattress on top of the other in the cot, as this may reduce the firmness of the sleep surface.
Babies should sleep on a firm, clean, well-fitting mattress & flat (not elevated) mattress.

There are some cots that have 2 levels for the base to be sited, but once a baby is showing signs of being to pull up on the sides of the cot, the base needs to be moved to the lower level.
https://www.productsafety.gov.au/standards/household-cots