Mailmerge can only use a single flat datasource.  I assume that what you
need to do is create two formletter merges, one for each worksheet, with
different content for each worksheet and then you want to create labels for
both letters.  What you should do, so that the label merge can be executed
in one go, is to create a third worksheet into which you copy and paste the
relevant data from each of the other two worksheets.

There will be an issue if you later add records to the individual worksheets
as you will have to remember to copy the records to the "label" worksheet.

On the other hand, if you have Access, you could have each of the original
two worksheets as a separate table and use a union query to combine them and
form the source of the label type mailmerge.  Then you would only have to
enter new records one time.