There's also no literature on the subject. So if you need benzos and opiates, there's zero guidance on what's safe. Just "we don't know, we don't measure, don't do it". In other words, abstinence.
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.]