I have not been having any problems so far with driving both 23’s of the same chip.
I recently replaced 2of the stepper drivers in my 3D printer. That was relatively easy because they were on their own board. I just had to adapt existing board to fit. on the gShield it might be a bit more challenging to figure out which pins to solder to, but it should be doable as the main board has to send those signals to the gShiled in the first place.
There are other controllers out their if you want to forgo the stock controller. For example there is smothieboard, which you can get with 4 or 5 drivers, so you can drive each Y motor on its own chip. (I believe it has a configuration mode for that)
My thoughts on feedback:
Yeas ago I upgraded my MaxNC cnc from the cheaper open loop system to a closed loop, thinking it would give me better accuracy and speed. This was a mistake.
The feedback circuit was prone to all kind of crazy RF interference from the RF nose cause by the brushes in the spindle motor. This would cause it to lock up all the time, especially on very long prints.
Yes high end professional machines have feed back circuits, but they also have high end spindles and tones of RF shielding.
I gave up on my CNC for years until I got a 3d printer. It is open loop and belt drive and runs at CRAZY speeds compared to my old MaxNC. It is accurate enough. So this made me rethink my old CNC.
Looking back on it I realize my main mistake was thinking that better accuracy = better quality. This is true to a point. But I have found that there are lots of other factors that will affect quality. Material, design, print profile, bit, spindle, etc.
And many of these have a greater impact on accuracy than any difference provided by open loop vs closed loop.
And unless I was doing professional jewelry or high end circuit board design I don’t think it would even be noticeable to me.
The other mistake I made was thinking closed loop = faster.
I later learned that close loop would compensate for missed steps. If my drivers are set right missed steps are not a problem. In the case of my old CNC I would have been much better of upgrading to a higher quality open loop controller, one that would not skip in the first place, than adding a feed back loop to adjust for missed steps. 
And, as in the case of accuracy, there are lots of other factors that effect speed. And in my case the biggest limit was the spindle motor.
I have learned my lesson. Yes on paper closed loop is better, but that doesn’t make open loop inferior.
Feed back, and limit sensors are nice but any spindle with brushes are going to make that a real problem to implement.
And unless you are willing to go with a brushless or compressed air driven spindle (options I was pursuing before giving up) I don’t think it is worth the hassle.
And for the materials and projects that I plan on using is it not a issue.
Good luck and good carving!
AronRubin