In a first database, I save all the invoices I get (PDF + Text). The name of the supplier is always part of the filename. For instance, all the files containing an invoice for my office have the word “OfficeHer”.
In a second database, let’s say in the group “Office”, I have all the correspondence related to my office, but of course no invoices. Now I would like to see in the group “Office” a list of all the invoices related to my office that are saved in the first database and contain the word “OfficeHer” in their filename.
A Smart Group would be perfect for that, but obviously it can’t search for files in another database.
There is no inherent risk to indexing a resource in more than one database.
However, you must be aware a change to the file made in one database will affect the file in the other database since they’re both pointing at the same file in the Finder.
Also, the same cautions about moving and deleting indexed files must be kept in mind whenever you’re dealing with them.