It's currently impossible to create a full TGV Duplex. Apparently Formation Designer thinks that the couplers between the power cars and the coaches are incompatible to one another. If you start with a coach, you can couple other coaches to it, but not a power car (yes, even if you use the correct end of formation coach). If you start with a power car, you can couple pretty much anything to it, with hilarious results, except for the TGV coaches...
