An interesting paper:
claims that "With the superior flushing of the 1.6-gpf water closet, the plumbing codes have prohibited the installation of a double sanitary tee or double fixture fitting for back-to-back water closets." They go on, and say that you can use a double combination wye and a 1/8 bend, or a double sanitary tee at least 30 inches from the centerline of the WC outlet. No explicit mention of the DFF, but it looks like the 30" rule mught apply there as well.
Disclaimer: I don't have access to an honest-to-God Code book, but it probably doesn't matter. What matters is what your local authority wants.