No, they don't cancel each other out. These two classes, Amphetamines and Benzodiazepines, work upon completely different systems in the body. It's, however, a fact that one of these drugs is a CNS depressant and the other is a CNS stimulant, so they will, to a certain degree, nullify each other. It's not uncommon for users of Amphetamines, including myself, to take Benzodiazepines to take the edge off. A little bit of Benzodiazepine can be great for reducing the anxiety and tension associated with stimulant use.
You might have the feeling that your stimulants are not working as powerfully as they normally would. This is a normal reaction, but if you prefer Amphetamines without Benzodiazepines, that's going to be a matter of preference.