At lowish doses and/or benzos with similar effect profiles, it is unlikely that you would notice that instead of 2mg alprazolam you've taken 1.5mg and 1mg lorazepam. As doses increase all benzos begin to become anxiolytic, sedative, and amnesic, so if you think it through I think in certain situations combining benzos is beneficial.
For instance, mixing alprazolam and lorazepam as the starter suggested sounds like something I might do for a stimulant comedown. Alprazolam is superior in anxiolytic strength so I'd take mostly that, but a bit of lorazepam for increased hypnotic effects and a longer duration to help me sleep later, as alprazolam wears off quickly. Really, for me, augmenting and finetuning your doses like this is the only real use for mixing benzos; they are interchangeable to a certain extent, but sometimes you need a longer duration (or a smooth comedown instead of coming off a big Xanax dose at once), or to add a benzo with muscle relaxant properties to otherwise mainly anxiolytic benzos.
This kind of tinkering implies knowledge and experience, so if you need to ask I would advise against it for the moment. The differences are subtle, anyway, so it's more likely you'll end up with a blackout.