I'm looking at it as a total system. You've got, say, a 12V battery with some internal series resistance. And, say, the two coils are 6 ohm each. Let's say the internal resistance of the battery is 1 ohm, arbitrarily.
If the two coils are in series, you'll have a total coil resistance of 12 ohms (13 ohms incl internal resistance), and will get 0.923A (12V/13ohm) through the motor, with a terminal voltage of 11.07V (12V x 12ohm/13ohm).
If the two coils are in parallel, you'll have a resistance of 3 ohms, and a current of 3A through the motor (12V/4ohm), and a terminal voltage of 9V (12V x 3ohm/4ohm). Lots more current, but lower terminal voltage.
If the two coils are in series, you'll have a total coil resistance of 12 ohms (13 ohms incl internal resistance), and will get 0.923A (12V/13ohm) through the motor, with a terminal voltage of 11.07V (12V x 12ohm/13ohm).
If the two coils are in parallel, you'll have a resistance of 3 ohms, and a current of 3A through the motor (12V/4ohm), and a terminal voltage of 9V (12V x 3ohm/4ohm). Lots more current, but lower terminal voltage.
Edit: formatting