jlebz:
Unfortunately it still seems like the motors RPM is far less than 19 RPM
When you have a problem the first step is to simplify. Get one motor working the way you want.
You almost certainly need to use acceleration if you want to get a high speed. You may also find that a motor that has nothing attached to its shaft gets caught up in oscillations. Maybe add a flywheel or a load on the shaft. Microstepping can alleviate oscillation problems but then you need a higher step rate for the same speed and the AccelStepper library does not have a very high max step rate.
...R