First observation is that there is no need to call writeMicroseconds() again after attach() if you've just called it before attach().
Second is that you have two different "stopped" values. Do your motors really have two different stopped settings?
A question is just how fast the motors are moving when attached this way?
Final observation is that all the comments I've made so far refer to the Servo library. I see that you are not using it. I'm not sure why. For the continuous rotation servos, at least, being able to vary the speed is inherent in the design of the motor, so using the VarSpeedServo class is unnecessary.