yeah ykm is right, mixing downers with downers is a very bad idea, and in alot of instances deadly, hell, poly drug abuse is just a bad idea, the only drugs i can really think of that are less dangerous to mix (however still not entierly safe) are caffene, weed and nicotine (although there has been evidence of nictoine inducing seizures/lowering seizure threshold with tramadol etc.)
really it becomes obvious that poly drug abuse is bad when you would be better off taking more addictive and harder opiates to get high than mixing 2 lesser opiates to get high. however if you are going to go ahead and do it anyways then make sure you reduce both your doses by quite a bit, as ever you can always take more but cannot untake what has been ingested, and yeah be very careful with APAP that stuff is pure filth.
all the best.