It cannot be done simultaneously.  It can however be done consecutively by
merging against one database and then changing the datasource for the merge
against the second database.

That's assuming that both databases contain the same fields.

If you were using Access, you could however create a Union query to combine
the two sets of data.

If part of the information for each record is in one database and part of it
is in another, in Access, you could use a select query to get combine the
parts of each record.