Searching across linked databases

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Originally posted by ashwken
An example of overlap could be found in a URD I have for Current Events, and a separate URD for Run Up to War. Both of these URDs are more research oriented, as opposed to cataloging, and as such could benefit from multiple db search. An arguement could be made to merge these two databases since there is some intersection of purpose, but on the other hand there is enough difference to keep them separate (Current Events is more general, where Run Up to War is more focused)

ok, just for the sake of discussing different approaches:

So the databases could be merged to a single one (where one can easily limit searches to only a part of database if desired). And with favourites and hoisting, it could really feel like you have two databases in a single file. Plus, if there are some things which are common, these could be connected (multiparenting) and promote understanding.

You say there is enough difference to keep them separate. Why would this "enough difference" imply keeping them separate? Are there completely different templates used for the similar things that would cause confusion?
Or is there a speed issue? DB file too big for OS to handle? Or some other compelling reasons? ;-)