After both girls get a clean bill of health from your vet and the little one is close to the size of the older one then you can begin intros.
If you do several play times with your girls in a tent or glider proof bathroom and they get along, go into the same pouch together and appear ready to sleep in the same pouch - I would put them into the new large cage at the same time. That way neither one will have marked the cage and they can begin their lifelong companionship together in the new cage.
Try doing intros during the day time when they are both sleepy. For me that has seemed to help them decide to share a pouch (have a clean one ready for them to share rather than the one either one has slept in.) If they are happy together sleeping in the new pouch - leave them together and be ready to keep an eye on them when they first wake up and eat dinner. Two dinner dishes might be a good idea the first few nights to make sure neither is a plate hog.
If the sleep together and do not fight over the food they should be fine together alone at night.