Well, I haven't been researching this a lot, but according to Bulbapedia once the White 2 player completes Area 5 of the White Treehollow, he can send the Treehollow Key to Black 2, in order for them to access the place.
So, you must complete White Treehollow - Area 5 in White 2 so you can send the Treehollow Key to Black 2, in order for your Black 2 player to access.
Hope I helped. :)
Source: Bulbapedia