if I want to have the same info (covers and fanarts included) basically I will have to download the new movie infos ALSO from the PC2
AFAIK, yes you will. I'm still working out a system for my multiple XBMC instances, but from what I've seen, if you want the fanart/images on the XBMC which did not originally scrape the movie, then you have to re-scrape on the second instance.
Someone please correct me if i'm wrong. Like i said, I'm still trying to work this out myself.
So far this central MySQL DB just adds nice features like "follow me" and keeping "watched" flags synced. It also syncs all the movie info (text-info), but you get anyway that when you have to re-scrape to get the fanart.
There was talk about storing the image URLs in the DB, to avoid this problem. But i'm not sure what that status is or if it still a viable solution