L293D Motor Driver Question

Hello guys, I am new to robotics, so I was wondering if any of you guys can help me out!

I am using L293D IC to control these two motors(https://www.sparkfun.com/products/13260#reviews). I am trying to figure out how much voltage I need to supply to the motors (VSS, pin 8 on L293D). For example, if I want to drive two 6V motors connected to the same motor driver, would I need to supply 12V, or do I need to supply only 6V ?(i.e. do I need to double the voltage when I connect two motors?)

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Also, in general, what would happen if I provide more voltage to a component than it actually requires? For example, connecting 12V battery to an arduino UNO, or connecting 12V battery to a 6V motor.

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Thanks in advance!