What I was planning was a rack and pinion at both sides and a tubular shaft running through the 4inch Alu gantry and belts from that to the pinions, the motor will drive one side of that shaft.
Hood
That's what I have right now but I want to change it for dual rack, dual motor when time / money permit.
One thing to consider is torsional twist, especially at higher cutting speeds with 180 degree direction changes. Seeing as you have a length of 3m you could do a torsion twist test. Get the torque specs of your motors (+ any gearing to factor in), and on the bench apply that torque to one end of the tube/bar while the other is held fixed. If that torque twists it, you will probably get that on your table during a cut, possibly leading to shaky oscillations and a scalloped edge cut.
Perhaps a thin wall high strength tube will be more rigid than a solid bar. I know aeroplanes and race car frames can be made from thin wall 4140.
I've messed up plenty with my first cheapy build so if I can help you avoid it, all the better.
Edit:
Oops, just realised Dave got there before me on the twisting matter. Great minds think alike Dave LOL.