Many gliders go through a somewhat rebellious stage where they tend to bite and test their limits with their owners. I is often referred to the glider 'terrible twos'.
For the nipping - a Ptsssst sound when you see your glider looking like it may be getting ready to bite will often stop it before the bite happens. Be consistent and your glider will learn that it is not acceptable behavior. If you do get nipped a sharp (not loud) NO or even an "Oww" will let your glider know you do not like the biting.
Another suggestion - if your gliders are new to you and have not yet had a well check up with a glider experienced vet you may want to take them to make sure there are no health issues or parasites that need to be dealt with. Sometimes gliders that have sudden behavioral changes are experiencing some health issue and do not feel well.
It is always good to establish your gliders as a patient with a vet in case they need immediate or emergency care in the future.