Try to take a good photo of each of their faces and top of their heads.
It is much easier to study a photo of a glider than one zipping around the cage and trading places with each other constantly.
There are usually differences in the head stripe you can identify - crooked on one side or the other or one may have a wider head stripe than the other.
Some gliders have more black around their eyes than others or black bars from the eyes to the ears.
Another good clue is the distance between the tip of the head stripe and the glider's nose - usually one's stripe will end closer to the nose than the other.
I also look at their tails. One may have a longer black tip than the other.
My first pair looked much alike as well but one had a very long black tip and pointed tail, the other's tail appeared a little more blunt and had a shorter black tip.
It will not take you long to learn their habits and personalities as well.