OldTatita:
I realized, that is possible to set both poles of the motor positive (HIGH).
Do you mean set both Arduino outputs HIGH (which is what @jremington is referring to)?
OR
Do you really mean "connect (say) 12v to both poles of the motor"?
Connecting the same voltage to both poles of a motor means there will be no current flow. For current to flow there must be a voltage difference.
...R