How do you write the code to run them both
You implement a processes like Bresenham algorithm Bresenham's line algorithm - Wikipedia, where both motors are stepped together ( or not ) after the appropriate delay.
and draws 330mA
That is normally per coil so for two motors you have four coils and so 330 * 4 = 1.32 Amps. Your power supply can not supply this much current and that is why you are seeing the juddering.
i could put that power supply in series with another one to get more voltage
Yes.
As far as i understand my max available current draw is still 0.5amps?
Yes.