Maybe these two were both mother-raised and therefore related better to each other.
No, same procedure as always at Wolong, cubbies were seperated from their mothers at the age of 5-6 months, so that they mothers could get in estrus for the next round asap. Yang Yang and Long Hui do know each other from the kindergarden at Wolong. They came to Vienna at the age of 2.5 years. After they arrived at Vienna, they were
not seperated! They could discover their new home together, there was no single moment of fear nor panic as there was shown by Hua Zui Ba at Madrid and Funi at Adelaide (and still can be seen at Adelaide!).
In Vienna the pandas are aloud to join each other when ever they want. The great enclosure construction with a lot of climbing facilities, bushes, caves and trees gives the animals also the opportunity to take a break from each other even when the gates are open.
Also after mating the pandas were not seperated. Any change in the daily routine could cause stress for the female. Only since a few days before giving birth, when Yang Yang started to build a nest in the den, the gates have been closed. Oddly enough, at that time Long Hui was not interested to move over to Yang Yang's area any more. Maybe it is instinct for the male's saftety to stay away from a mum with her cub.
When the new cubbie will be old enough to stay on its own in the den for longer time, Yang Yang and Long Hui will join each other again. It worked great with Fu Long, Mummy Yang Yang spend some time with Long Hui and for her own while Fu Long was sleeping, the other time she was a great Mum and took care of her baby. I'm sure it will work with the new baby too.
IMHO beside several other factors (e.g. food, medical training, general dealing with the pandas etc.) there is a main reason for the success at Zoo Vienna: a wonderful enclosure that absolutely fits the needs of the animals, with the focus on the needs of the animals and not of those of the visitors!