I'd like to use a worm drive to achieve the same results as pinion gear with a rack on either side. namely that the rotation of the worm gear would cause one rack to move left and the other to move right. is it possible to have one thread on the worm gear and create right hand thread on one rack and left hand thread on the other rack?
No, this is not possible. As a worm screw rotates, the 'tooth front' moves along its axis, this is the same on the top or the bottom of the worm.
The Worm Wheel is often not cut 'handed' at all.
While it would be possible to 'straighten out' the wheel into a rack, it would not be possible to get opposing linear motion from a single worm screw.