I was in a similar situation to you. I had an older model Dynatorch machine that was driven with a single X axis motor and a cross shaft system. The cross shaft had a pinion on each end riding on 2 table length gear racks.
The X axis motor had its own gear rack. There were several things with the cross shaft system that I did not like and wanted to improve.
The cross shaft system;
1. The cross shaft gears and rack were very noisy
2. It added extra rolling resistance and vibration
3. It added even more weight to the gantry
4. And the biggest problem was the accuracy of the cross shaft system. In theory the cross shaft pinion gears should stay in sync with each other keeping the gantry straight when traveling from one end of the rack to the other.
On my table I found this not to be the case. When I checked my cross shaft travel, I determined that one side would travel about 1/4" further than the other when going from table end to end. (5 x 10). I am sure it all boils down to gear quality, wear, and back lash.
I decided to upgrade to dual x axis drive motors among many other changes I made to the machine. I have been extremly happy with the performance of the machine with the added X axis slave motor. It is much quieter, smoother, more powerful, and the acceleration is excellent.
The added X axis slave motor will also add rigidity to your gantry. Depending on the condition of your existing X axis gear rack you might consider buying 2 new racks and pinion gears to maintain the accuracy between the master and slave sides. The only other consideration is the addition and extension of the wiring harness to the X slave motor. To answer your question about binding on the slave side; You won't have to be concerned about rigidly guiding the slave side only the master. You can retain your existing roller support wheels which will compensate for slight misalignment in the X master and slave sides.
With a little reading you can easily figure out what your existing gear rack is. You might check out this gear supplier. They were very helpful when I contacted them and they are one of the suppliers who stock 12' lengths of gear rack.
Moore Gear Website
Here is a link to my build; look at the pictures in thread # 82 and 121
NEW BUILD / Dynatorch Upgrade