Its not about taking them together,its about taking them in the right order.
Popping all the piils while sniffing coke the same time,would result in a messed up feeling(not necesairily in the good way)and prolly youd just nod,and maybe feel uncomfortable.
Now,if you take a couple of hydros and do some coke,you might get a nice speedbally feeling,especially if you have tolerance to opiates,the coke will be very noticable,so id recommend to keep the benzos for later,when the speedball starts to wear off...