I have experience with this kind of setup. If you go to this kind of setup you can’t tension the belts the way shapeoko was intended. Both belts are fixed at both ends, with the top belt having a “loop”. The motors are attached in some slots so that they can be moved to create the necessary tension. I have been thinking of doing this myself as i tend to think that it would greatly reduce belt stretching and increase belt life.
However as someone i think mentioned earlier, ball screw is probably the way to go. Also there would be concern for there being enough clearance for the second belt. The v-groove rollers and the bearing rollers are really close together so there is the worry that the bearing rollers could start riding “on” the belt and cause some serious binding.
Initially i had wondered why shapeoko hadn’t done this in the first place as it would be a huge advantage over the current setup. However it also make the adjusting of the belt tensions more complex and would definitely cause people more grief.