Actually Jnaas2,
A double fixture fitting shown as #500 on page 30 of the Charlotte Pipe Co. PVC/ABS Dimensional Drawing Catalog linked below should be used instead of a double sani-tee.
http://www.charlottepipe.com/Documen...e_Fittings.pdf
I am somewhat concerned about Freddiy's vague description of tying into a horizontal line though and I have visions of it not being anything as nice as the picture you posted. An isometric sketch of what he's doing would be very nice but even a rough sketch would be nice before we stamp an approval on the project.