You can use a catalog (or in Word XP and later, it is called "directory")
type mailmerge main document in which you arrange one set of mergefields in
the configuration that you require in a single cell or single row table,
with the height of the row such that only two rows will fit on one page.
Then when you execute that merge you will get two records to a page.

Alternatively, use a formletter type main documnent in which you insert a
two row table, once again with the row height such that only two rows will
fit on the page.  This time, you will need to insert the merge fields into
each row of that table and in front of the first mergefield in the second
row, insert a <<Next Record>> field.