Howard Chu:

Indeed, the idea of "multiple suffixes" is that you're using suffixes with no subordinate relation. However in that case, there is no logical reason to keep them all within the same database either. They represent totally independent namespaces then, and keeping them in a single database offers no advantages.

Well, it offers some advantages. Some features don't work between multiple databases, like subtree rename and (yuck) aliases. Also it can be simpler and less work to administer and tune one database than several. Though instead it can be more work for slapd, e.g. index data both suffixes get joined together, indexing can give more false positives which will have to be eliminated by checking the filter.