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.