Thank you for the replies @johnwasser
If the input and the output units of a PID are the same. That means I need to convert the output of the first PID to angular rate/velocity.
And as you said, the pitch, roll and yaw values will affect the thrust of each motor. Since the output of the second PID is an angular rate and the throttle is having a different unit (could be time or percentage), how can I get unified unit? or How can I get from an angular rate to the throttle's unit?