I had read before that torque drops with speed, but never really comprehended the magnitude of it. Look at this chart I was sent: http://blog.misumiusa.com/wp-content/uploads/2015/02/Stepper_Server_Comparison.jpg
At idle it is just about 300oz-in, but at just 5 revolutions a second (at 8mm a revolution that would be 40mm/s) at 12V instead of 300oz-in it is just 100oz-in, but at 24V it is still 250oz-in. If you need more torque or want to go faster wouldn’t it make a lot more sense to modify your electronics to accept 24V rather than bumping up current?
I would think series wiring would just amplify the whole torque over speed at 12V drop because of the additional resistance of the extra wiring to the other stepper.