Do you have anything in the interrupted function of the first command? The command itself may be ending but whatever it's doing may continue unless you explicitly tell it to stop in the interrupted function. How do you know they both keep getting called in parallel?
Here's some general command based documentation, but it has some specific detail about interrupting and requires():
http://wpilib.screenstepslive.com/s/...uling-commands
http://wpilib.screenstepslive.com/s/...ed-programming