You can use each of the two "mail" lists as the datasource for a Directory
Type mailmerge in which you set up the main documnet with a one row table
with the mergefields in the cells of that table.  After executing the
merges, copy the table from one document and add it to the other so that you
end up with a single table containing all of the data and then insert a row
at the top of the table and insert field names in that row.