I mix diazepam and alprazolam all the time. I actually prefer taking combinations of different benzos rather than taking them individually if I'm looking for a recreational effect. Other than the standard warnings of tolerance and addiction, there's no danger in using different benzos concurrently.
@Albion, the combination would not be fatal. Benzos are extremely safe unless combined with other types of depressants like alcohol, opiates, barbiturates, GHB, etc.