TBH though guys, it isn't really a problem just driving the extruder carriage the way I do. i.e map the drives so that the Duet treats both gantries as one. Like I said, I have around 4kg moving in Y at 1,000 mm above the floor, so a very high centre of gravity and it really isn't an issue, even the short zigzag moves. Just a very slight rock if the feet aren't levelled and there is some "give" in floor. Even then, the entire printer, including the bed moves as one so I haven't noticed any artefacts with the prints. I guess it might be an issue if the frame wasn't rigid but it all seems like a lot of complication to get around a problem that doesn't really exist IMO.
Edit. Another approach might be to use flexible drives like the nimble for example, and stick with a single gantry. My pockets aren't deep enough for me to now abandon my 5 E3D Titans and replace them with 5 off Nimble flexi drives. Actually, I did briefly look into it but getting them to fit a diamond hot end was just going to take up too much X and Y travel for my liking.