Can you be more specific? Do you basically have a data source that is split
between two Excel documents, or what?
In most cases the simplest thing to do is either to have more than one merge
(e.g. attach your mail merge main document to excel file 1, save/close, copy
your mail merge main document and attach the copy to excel file 2, then do
two merges), or to combine your data sources manually before merging. There
are other possibilities, but they mostly require VBA.
Peter Jamieson