Many (if not like more than half, I forget the statistic..) of those overdose deaths result from combining a GABAergic like alcohol or especially benzodiazepines with an opioid.
oh, and 4. combining a stimulant with a larger/more euphoric dose of the CNS depressants so if the respiratory stimulating effects of the stimulant run out first, respiratory depression ensues
I didn't understand why people mixed in benzos, unless to curb withdrawals.
Some people take them to increase the nod. Others might have insomnia and don't want to take so much opiates to really sleep.
Edit: And there's obviously some information on this, somewhere because e.g. IV opiate+midazolam is quite a common thing. [In a 3rd world pharmacy, I was actually offered liquid midazolam as a sleep aid when I complained long-lasting benzos were too long-lasting and asked for a medium-duration sleep aid.]