Almost all the Reprap electronics drive both of the Z steppers in parallel.
Part of the reason is that due to the way they are driven, if one fails to step, the other ALSO fails to step, so your Z usually stays level. If you drive them independently, and one fails to step a few times, your system will be unbalanced and could lead to binding.
Q: What are you trying to achieve by using twice as many steppers? The issue of build volume is usually speed. The reason it takes longer to print a larger object (ie: scaled up) is that the amount of travel distance you need to perform with the head is cubed, and it's very hard to push plastic out of a nozzle very fast in a quality way, so you tend to have to slow your printer down. You usually don't need that much power to require more than 1 stepper to drive an axis unless you have a VERY HEAVY machine. The only reason most designs use 2x Z motors is because a second motor is about the same cost as using a bunch of pulleys, bearings and a belt to connect both (or more) Z rods. It also avoids the need to allow for the need for space for the belt between the 2x Z's.
By using more than one extruder, you can however at least double the amount of plastic you push out. There are designs that use dual extruders on the same X axis (independent movement) that could print two halves of the same object at once (if you can avoid collisions around the middle, which the slicer 'may' be able to do for you).
If you were to try and do dual (or more) extruders using both X and Y using bed that moves in Z design (like the QU-BD that you linked), apart from the issue of avoiding any potential collisions (which would grow by a square law), you would need a way to try and make the extruders avoid all the mechanics for the other extruder (eg: any gantries).
Part of the reason is that due to the way they are driven, if one fails to step, the other ALSO fails to step, so your Z usually stays level. If you drive them independently, and one fails to step a few times, your system will be unbalanced and could lead to binding.
Q: What are you trying to achieve by using twice as many steppers? The issue of build volume is usually speed. The reason it takes longer to print a larger object (ie: scaled up) is that the amount of travel distance you need to perform with the head is cubed, and it's very hard to push plastic out of a nozzle very fast in a quality way, so you tend to have to slow your printer down. You usually don't need that much power to require more than 1 stepper to drive an axis unless you have a VERY HEAVY machine. The only reason most designs use 2x Z motors is because a second motor is about the same cost as using a bunch of pulleys, bearings and a belt to connect both (or more) Z rods. It also avoids the need to allow for the need for space for the belt between the 2x Z's.
By using more than one extruder, you can however at least double the amount of plastic you push out. There are designs that use dual extruders on the same X axis (independent movement) that could print two halves of the same object at once (if you can avoid collisions around the middle, which the slicer 'may' be able to do for you).
If you were to try and do dual (or more) extruders using both X and Y using bed that moves in Z design (like the QU-BD that you linked), apart from the issue of avoiding any potential collisions (which would grow by a square law), you would need a way to try and make the extruders avoid all the mechanics for the other extruder (eg: any gantries).