It really depends upon what you want in the output.  If you want to pull
data from two tables, you would use a select query to link the tables on a
common field and use the query as the data source.

If you are talking about a "multiple items per condition (=key field)"
mailmerge, Word does not really have the ability to do it and I would use a
report in the database instead.  However, see the "Group Multiple items for
a single condition" item on fellow MVP Cindy Meister's website at:

http://homepage.swissonline.ch/cindymeister/mergfaq1.htm#DBPic

Or take a look at the following Knowledge Base Article:

http://support.microsoft.com/default.aspx?scid=kb;en-us;211303

Or at:

http://www.knowhow.com/Guides/CompoundMerges/CompoundMerge.htm