you can't
when the autopilot is ON, you can't make a full speed brake selection. yes, you can select the speedbrake lever to be in full detent. but the system sure would limit the spoilers to half. (not exactly half). that's because ;
when it comes to a roll control function and a deceleration funtion, the roll function has the priority. so the system limits the deployment of the spoilers. In emergency descent you have to disengage the autopilot and select full speed brakes.