Research chemicals that are most lethal in combination?

I'm someone who feels that preparation is important. I doubt Ill get much help here, but I want to know what substances for sure to avoid taking together. Id assume psychedelics for the most part are fine, but what about something like that opiate analogue (U4471 or something). Is it like other opiates that if you combine it with a benzo (say etizolam) it could cause problems?