Is it common to use "A" and "B" to represent something during talking?
E.g. I would like to say "Lets call (someone from some department) A, and (someone from some department) B. A and B are....".
Is it common to do it in English?
I have heard substitute names used more often, plain names like "Sally" and "Jim", but "A" and "B" would not sound strange at all. However, you may want to call them "person A" and "person B", at least the first time you mention each.