About your first question:
Code:
public double getJoystickValue(Joystick joystick) {
if(Math.abs(joystick.getValue() < 0.1) return 0;
else return joystick.getValue();
}
This checks the value of the given joystick and returns a double based on your threshold. You'll have to create the Joystick objects yourself; I am unfamiliar with Xbox controllers.
As for your second question about using two joysticks, the RobotDrive classes have an arcadeDrive method that has two arguments: the speed you go forward (the value of the throttle joystick) and the speed you turn at (the value of the turning joystick).