I've heard that some people have done it, but not using your methods.
8ft is really too long for acme screws, unless you used really big ones, which would then require really big motors too spin them. I believe that the people who have done it have use rack and pinion. Apparently this info is available on Joe's forum to anyone purchasing plans.
If you link two motors together mechanically, they'll always be fighting each other a small amount and may not give you much better performance at al. Better to use a single larger motor.