If both motors are at velocity 0 and you set both to go to a set velocity, the second motor that you set will not start moving until the first has reached that velocity - but you can definitely have two motors moving at the same time, as this only happens when the motor velocity is changing (at the rate of acceleration).
At high acceleration, you won't notice because the amount of time that the motors spend changing from one velocity to another is minimal - but if you use low acceleration, where is can take seconds to change from velocity 0 to velocity 100, then you'll notice.
If you do have an affected 1064 and would like it replaces, just contact
support@phidgets.com.
-Patrick