The database servers don't need to be on the same host in order to connect to them. It's possible to make use of the database without losing any modularity on AUR2's part.
Actually, it isn't. You *would* lose modularity, and you would have even tighter coupling. Additionally, remote sql invocation over arbitrary remote networks isn't the greatest idea from both a performance and a security standpoint. The goal of the AUR2 should not be tighter coupling.
I was talking with Dan about "standardising" the main site and AUR, mainly in terms of Django models, because the two sites are so similar. AUR could easily simply be a Django app that the main site uses. It's a bit off-topic, but I don't think integrating the sites in some way (at least to be able to duplicate code easily) would be a bad move.
With the rearchitecture of the main site, I don't think this is very feasible. I strongly feel that the AUR should remain "it's own app".