The preferred approach is two separate motors and drives, slaved in software.
However, you may have quite a few problems.
What drives are you using?
Unless you use a Gecko G201 or G203 at around 70V, you won't get a lot of speed out of those motors. Coupled with 1/2-10 acme, that means you won't get much speed out of your machine.
In addition, the screws will whip badly at that length, which may limit you're speed even more.
You're best bet for a 6' drive system is rack and pinion.
Will the motors have enough power? Depends on the power supply, drives, and how fast you want to go, in addition to the drive system choice.