If you have only just started the alprazolam you should get away with it. But if you have been taken it for a prolonged period of time I wouldn't just stop one and start the other. That might be a bit too risky. With withdrawls felt. This is a question your doctor should be able to answer for you. Prolonged use may be a slow switch over. To just up and stop alprazolam may cause more harm than good.
I would talk to your doctor first. These medictions are in different classes. One is a benzo, used on an as needed bases and the citalopram is an SSRI. In my experience, many doctors will prescribe a benzo when you start an SSRI. SSRIs can take weeks to 'kick in,' and for one to start feeling its benefits. Did you just start taking citalopram?