Have you investigated using Terracotta / Compass? We need real-time updates across the index using multiple web servers. I recently got this up and running and we're going to be doing some performance testing. It's very easy, essentially you just replace your FSDirectoryProvider with a TerracottaDirectoryProvider that wraps the compass TerracottaDirectory.
http://relation.to/Bloggers/HibernateSearchClusteringWithTerracotta