Adding a citation after paper is accepted I am a junior researcher in mathematics.
I worked simultaneously on proving Theorem A and Theorem B. They both generalize Theorem X in two different directions. Theorem X was proved several years ago (not by me). The proofs of Theorem A and Theorem B use very different techniques, and so I decided ahead of time to write one paper for each.

I finished proving Theorem A before I finished proving Theorem B. Then, I submitted Paper A to a journal (and the arXiv) proving Theorem A. Paper A includes a survey of related problems in the introduction. After a short while, I managed to prove Theorem B, and two months after the submission of Paper A, I submitted Paper B (to a different journal, and also to the arXiv). Now, six months after the submission of Paper A, it is accepted with requests for minor corrections. There is no response yet regarding Paper B.


  In this situation, is it a reasonable/common to add one/two sentences to the survey in introduction of Paper A, explaining and citing the arXiv prepreint of Paper B, in addition to the minor corrections the referees asked for? (and letting the editor and referees know that I did that in my answer to their reports which comes along with the corrected manuscript).


Note that I am not asking if it is ok to cite a preprint. This is something I do, and so do other people, at least in math. My question is about making a change, after acceptance, which was not asked by the referees and the editor.
Of course it's ok. Even if the paper is accepted, but some minor corrections are required (I guess the reviewer noted that after these corrections the paper can be accepted without sending the revised manuscript to them again), your work on it has not not yet finished, and you are free to make modifications that you consider appropriate - not only those that the referee asked for. So if you found something that the reviewers didn't ask for, but adding it to the text improves the quality/completeness/etc. of the paper, you definitely should do it. 

Talking in general: in the meanwhile you could obtain some new data, could perform the experiments on different equipment, apply another method to confirm/strenghten the results, or some of your colleagues pointed out you have some minor error or that some part is unclear, or you found some other relevant works that you want to compare your results to - all of this can be done even in such an almost-accepted stage.

Finally, it could also happen that you'd need to add some extra sentence to keep your overall style consistent after the requested corrections, or to make a more natural transition between two parts of the text; this would also be perfectly fine, even if - strictly speaking - that's not precisely what the referee asked for.

Overall - aim at the highest quality of the article that you can achieve at the moment.