I can give you a HUGE hint here: Turning a ballscrew that long, which has to have a large diameter has huge torque issues. There is a reason that large machines use rack and pinion, it is torque issues and whipping. With a ballscrew that large, we were lucky to get 200 IPM, if that. Truly state of the art machines these days are using linear motors.
As for his comments about a dual motor drive and timing issues? He obviously knows nothing about the electronics or the speed of electrons, does he? Drive a large format machine down the center with a screw and you can watch it twist on the bearings (unless you are doing a moving table with the bearings about 8' apart)
A general comment: There are some people in this world who can only feel good about themselves by berating, putting down, and citicizing others. They rarely accomplish little themselves, because they are themselves afraid of being criticized. They are to be pitied, life for them is miserable. They never get to see a friend smile when they say "Wow, that looks good, or what a great idea, give it a try!"
So my advice to you? Just build your machine, have a good time doing it, and to hell with the critics.