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?
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.