Different benzos, allthough all acting on GABAa, each have their own properties (acting on GABAa sub receptors i think, one to six), some are mainly hypnotic, some anxiolytic, some muscle relaxing, anti-epileptic some are short acting, some are long acting, half lives vary etc. So if you know what effect you are trying to achieve, mixing benzos can be great, on day to day, i'm fine with just some ethyl loflazepate (victan) for my anxiety, but for a come-down off cocaine or MDMA for instance, valium, temazepam, loprazolam or nitrazepam for instance will be great to relax your muscles, and get some sleep.
eg. You're coming down off a stim, you might want to take a xanax to take the edge off (anxiolytic mainly) then take a valium 30 minutes later to relax, then later take a hypnotic ie. loprazolam to sleep... or you can take all three at the same time, which depending on your tolerance will send you straight to sleep, or put you in an extremely relaxed state.
Or just for benzo effect i would with what i have, combine victan (anxiolytic), with prazepam (weak but covers most of the benzo effects) with valium (very muscle relaxing and pleasant) and maybe pop a loprazolam into the mix, hypnotic and muscle relaxing, to achieve an all over benzo high, however i don't do this, only for stim/psych come-downs...