There are 3 values for the option DUPLEX= in the EXPR column of the FRX, objtype=53:

1= Simplex reporting - i.e. the normal
2= duplex flipped on the long side of the paper - this is the normal
3= duplex flipped on the short side - also known as tablet mode - not sure why.

You may find it easier to drop an option group for this into your "report options" form and then manipulate the EXPR column yourself. This depends really on your environment.

HTH,
Simon