With 2.8V 1.68A motors and a 24V PSU, connecting all 4 Z motors in series should work well, and better than a series/parallel combination.
You will probably run your motors at 1.2A or less, much more and the drivers will overheat (even 1A is probably enough for reliable operation). But the motor current draw from the 24V supply will be much lower than this. With the motors at rest, the X and Y motors will take about 150mA each on average from the 24V supply, and the four Z motors in series will take around 600mA. So about 1.2A for all 8 motors. That's the magic of chopper stepper motor drivers. The current will increase when the motors are moving, but is unlikely to exceed 2.5A. Each hot end will take less than 2A if it is a 24V 40W heater. That leaves the heated bed. For dual extrusion, I reckon you need at least a 300W power supply, assuming a standard 120W bed heater.
You will probably run your motors at 1.2A or less, much more and the drivers will overheat (even 1A is probably enough for reliable operation). But the motor current draw from the 24V supply will be much lower than this. With the motors at rest, the X and Y motors will take about 150mA each on average from the 24V supply, and the four Z motors in series will take around 600mA. So about 1.2A for all 8 motors. That's the magic of chopper stepper motor drivers. The current will increase when the motors are moving, but is unlikely to exceed 2.5A. Each hot end will take less than 2A if it is a 24V 40W heater. That leaves the heated bed. For dual extrusion, I reckon you need at least a 300W power supply, assuming a standard 120W bed heater.