Duplexing is a printer option rather than something set in the Word page
setup dialog. Sorry.

Why not have the first letter print duplexed also?  Just have the second
sheet a blank page.  You should be able to do that easily with section breaks
and not have to do returns to the bottom.  I do that all the time on my
Laserjet.

You *may* be able to achieve this with PRINT fields on the relevant pages to
switch duplex on and off - if your printer driver accepts PCL commands. A
PRINT field is simply a type of field that allows you to send instructions
directly to the printer.

There are 3 possible data values for duplex setting:  PRINT 27"&l0S" simplex
PRINT 27"&l1S" vertical duplex . PRINT 27"&l2S"  horizontal duplex
(bookformat)

The codes after PRINT are 27 (escape), double quote, ampersand, lowercase L,
0, 1 or 2, uppercase S and finally terminated with a double quote.

Thus for horizontal duplex you would insert the following field :
{ PRINT 27"&l2S" }