They can
absolutely reach the back of their own head and over groom! Self Over grooming can even happen when they have a cage mate.
Bald patches are usually from their grooming nails, not from licking like a cage mate would groom another glider. If a cage mate is licking an over groomed spot it's possible they are trying to help stop the over grooming by leaving a little of their scent on the patch.
Our Simon came to us with two large bald patches on the back of his head. He had been a single glider for 4 years until he found his way to a rescue where he began over grooming. Just the change in environment was enough to make him over groom.
Once we adopted him and he settled in he stopped over grooming and his hair grew back nicely. Part of Simons thing is he will hide himself away when he feels stressed or overwhelmed.
I had to remove some toys that he hides in to get him to be more active at night. That seems to help the most. A small uncovered ball pit instead of our large covered one, the sleeping pouch is removed until I go to bed around midnight. With out having these hiding places he is much more active and participates in playing more instead of over grooming. Once he stopped over grooming and his hair grew back I gradually started putting the hiding places back in.
However, We recently upgraded our cage, and he began over grooming the same spots. He has a cage mate so it's not from loneliness, they get along famously no fighting or arguing. The cage has toys lots of toys that are switched up and changed regularly. The same fleece cage set with his scent on it is in the new cage etc...
Again the change in environment, even though it's a nicer cage and the cage is in the same place the old one was, it is still change for him. We are his 3rd family in a relatively short amount of time. The rescue only had him about a month and half before we could adopt him.
He has had to deal with a lot of changes, new cages, being around several colonies while still being alone, being introduced to Leela, another new house and cage, being introduced to toys ( he had never had toys or even a pouch before ) and yet another new cage. I am sure it's overwhelming to him experiencing all these new things he's never had before.
I never see him over groom but the patches are visible. He eats very well, he's social with Leela, social with us. He loves out of cage time.... for about 20 minutes then he is ready to go back in the cage. He doesn't use the wheel as much as I would like but the new raptor wheel is being shipped today I am hoping it will be easier for him to use.
So my advice is watching his behavior, if he hides a lot remove some of the hidey spots. Consider any changes you have made to their environment....
And my questions to you are
1 how big is their cage
2 do they have toys
Sometimes over grooming is due to inadequate space and or boredom. Though with Simon that is certainly not the case the cage is large and plenty of things to occupy them... So this is just a question as I don't know how you'rs are housed.