CNS depression if severe can potentially cause :
"reduced heart rate. low breathing rate of fewer than 10 breaths per minute. extreme confusion or memory loss. nausea and vomiting. poor judgment. blue lips or fingertips. irritability and aggression. clammy or cold skin. low breathing rate of fewer than 10 breaths per minute."
In milder cases symptoms might be:
"feeling sleepy and uncoordinated, staggering, blurred vision, impaired thinking, slurred speech, impaired perception of time and space, slowed reflexes and breathing, reduced sensitivity to pain."
Everybody responds differently to medications and so some people may be able to tolerate combinations like this. A doctor knowing the patient's history might prescribe these medications if the benefits outweigh possible risks.
Best regards and I hope this helps.