Yes you can carry both gliders in the same bonding pouch.
Since they are sisters, you will not need to quarantine the second glider when you bring her home, but if you do not bring her home quite soon, you may need to re-introduce the two gliders to each other which might require a temporary cage for the new glider until you have had some time to swap pouches and do supervised introductions before putting them in the same cage together.
They are likely to get along just fine, but if they have been separated for several weeks, they might fight if just put together in the cage.