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One motor is probably sufficient for direct drive extruder. Someone already wrote in comments under http://www.thingiverse.com/thing:200078 that he have dual motor direct drive extruder you described but I did not find the model for it yet.
With 12V version of 28BYJ-48 you will need at least 24V PSU (probably even 35V) due to resistance and inductance of the joined coils! Try to avoid 12V version at any cost.