They don't technically cancel each other out, but they do counteract the subjective, desirable effects "feeling high"..
Benzos aren't really CNS depressants, but they do potentiate the CNS depressant effects of some other drugs.
If you take benzos on top of amphetamine, it can overpower the feeling of the amphetamine, and vice-versa...but it's important to remember, that benzos don't directly diminish the stimulant effects of adderall, so taking too many benzos could cause some people to redose with a dangerous amount of adderall..
The combination of both drugs isn't strictly contraindicated, and are prescribed together often...so, in reasonable amounts, it's not a particularly dangerous combo...
In practice, the perceived "canceling out" effect of these drugs can lead to people redosing high amounts of both, trying to "balance out" the high....and that can cause all the problems normally associated with taking high doses of both drugs...fast heart rate and tingly and jittery from the amphetamines, and loss of coordination, mood changes, memory loss and blackouts from the alprazolam...
Take care when using this combination, but have a set limit on how much you're going to take of each in the beginning, and dose low on the benzos or you won't feel the Adderall at all...
This is a combination I personally enjoy, but I've had difficulty finding the right dosing balance in the past....