I believe we've answered this at this point. Succintly, these two classes, Benzodiazepines and Amphetamines do not "cancel each other out" because, although one is a stimulant and one is a sedative, they work through different mechanisms in the body, so you'll actually be experiencing the full-force of both drugs, just interpreting them differently. It shouldn't be a particularly dangerous practice either, so long as dosages are kept reasonable and there are no other drugs in play.
