Seems like twice as many zip ties for folks to complain about. IDK. I thought these belts were supposed to be pretty tough to stretch. I’m guessing it works pretty much like a spring, so the deflection is proportional to the distance and the force. It will probably just change that stretch by 1/2 with twice the belt (if they are both configured the same). If the belts are really an issue, then 2x might not be enough of an improvement. Maybe there’s a better way to reinforce them? Are there belts with steel inside them, or made from a serpentine belt?
At some point, the answer is just, c’mon! It’s an 8 foot long machine! Is 1/8″ going to kill you?
What do you set your accelerations to? I know they are detrimental to printing fast, and to lasers, but for CNC’ing, reducing them gives a perception of quality, and will reduce the force on the belts. I was just messing with mine, and the defaults for grbl are set to 10mm/sec^2. I set mine to 100mm/sec^2 for xy and 10 for z. I have no idea where I got them from, but I was hopefully smarter then.