I've taken these at the same time over the last year - no side effects whatsoever. Apparently citalopram has anti-anxiety/panic properties but on its on and in isolation, for me, citalopram has done nothing to help in this respect. Propranolol, on the other hand, works very well in terms of relieving the symptoms of anxiety and panic. My dose of citalopram was and is, 20mg per day, and I use very little propranolol when I need to, as I find I only need a very tiny dose of it to help me, e.g. a quarter of one tablet.
For me, citalopram and propranolol haven't clashed whatsoever.