I just finished testing exactly this code on the EDU controller. It works fine as long as there are no conflicts with the values that your fast and slow code sections output. I wrote the code for the fast section, and forgot that the slow section would output 127 every 26 msec
The edu motor sounded really weird as a result. As soon as I figured this out though everything worked.
I am not sure if updating the values that quickly will actually do any good, however. Even if the speed controller can respond this quickly to the change in PWM, the motors and the drivetrain certainly won't. Inertia in the drivetrain will erase all difference between fast and slow updates, I think. If I'm wrong please correct me on this.