1. Google "wiring stepper motors in series".
2. Plug-in drivers have inadequate cooling for the driver chips, and already struggle to provide enough current to the stepper motors in many systems. If you connect two motors in parallel, then they need double the current to produce the same torque. Whereas if you wire them in series, they need the same current but double the voltage. The stepper motors we normally use only need around 2 to 3V unless they are moving very fast, so if you connect them in series that's 4 to 6V, still well below the supply voltage of 12V or 24V.
2. Plug-in drivers have inadequate cooling for the driver chips, and already struggle to provide enough current to the stepper motors in many systems. If you connect two motors in parallel, then they need double the current to produce the same torque. Whereas if you wire them in series, they need the same current but double the voltage. The stepper motors we normally use only need around 2 to 3V unless they are moving very fast, so if you connect them in series that's 4 to 6V, still well below the supply voltage of 12V or 24V.