They aren't actually running simultaneously. I'm not sure if I'm interpreting "preemptive time slicing" correctly, but any Command-Based program could be written in the IterativeRobot style with a large number of if statements
. They're all running on one thread as I understand it, and I am not sure you have any guarantee to the order they run in. That is why Commands can declare that they require exclusive access to a Subsystem to tell other Commands that they will be writing data to it.
Both commands will run the execute() method in their class each robot loop (usually 20ms) until their isFinished() check (also run each loop) returns true. The Scheduler takes care of all the behind-the-scenes work of maintaining command life-cycles.