Correct me if I'm misunderstanding what you're asking, but I think the arcade drive method in wpilibs RobotDrive class will do exactly what you want.
http://team2168.org/javadoc/edu/wpi/...ouble, double)
There's some code examples here you could modify:
http://wpilib.screenstepslive.com/s/...botdrive-class
I'd type up an example, but I'm on my phone. Let us know if you need some more guidance.