well, first things first (those that SWIY or whoever else here thinking about such kind mixes must know) - benzos withdrawals can be much harder than an opiate withdrawal so before doing anything with benzos think about this.
now, the euphoric effect of the benzodiazepines is going away pretty quickly and this is the reason why most people are addict and they are not feeling nearly anything else only except for the hard pain when they don't take their dose. but in regards of the 'main' question or what I have understood to be the main question - the heavy opiate users (addicts) do like benzos due to the fact that they are a good ersatz when the opiates are not on the street and they need to get their dose or to take something similar to keep the withdrawals under the control.
there are some users with a lot of experiences, mixed the both substance, heavy trip. this is a very spread thing with the methadone thing and there's even an movie that from as much as I remember it is called Methadonia - in that movie they are talking about this phenomenon that occurring in the US and from as much as I think - in a lot of other countries either.
when that person who is being addicted to opiates is not able to find the dosage that he needs, the he also can use some zolpidem either, zopiclone an imidazopyridine sub division or assimilate to the benzos (which is a mistake if you ask me). the opiates is a dangerous way. the benzodiazepines are dangerous either but now… taking them both together?! I guess there is no reason for me to explain… they are heavy and extremely dangerous - you should stay away from them.
I also know people who use them both but instead of taking them together which is so dangerous, he prefers to alternate, 2 or 3 week on opiate and after those weeks, the exact same amount of time on clonazepam or valium or such stuff. in this way he says that he is able to avoid tolerance and addiction making them work longer and better.
while I have been in France, I have read there an article, in a very objective and anti prohibition web papers, about a man, when he did not find opiate, or simply finding skenan (sulfate morphine). that man was trying to find clonazepam drop bottle, a strong bear, like 8.6 Bavaria and to add 110 mg of the clonazepam on his beer and then to slowly drink it, for sure that help with the opiate withdrawal that he had.