My manuscript was sent to a Journal A and was reviewed and finally rejected but with encouragments to resubmit later after having addressed some concerns that are detailed by the Editor and reviewers. As we had time constraints and we thought that one critical experiment could not be addressed easily, we sent immediately the article to a Journal B. During the process of Journal B reviewing, we addressed the concerns of Journal A as they are relevant for the study and we had the good surprise to address all of them including the tricky experiment. Then, Journal B sent us back a notification that the paper is interesting, not accepted in its present form, but they give us 4 months to submit a revised manuscript (with limited concerns that we had already addressed for Journal A without even the tricky experiment required by Journal A).
My question is: can I resubmit a revised manuscript that address all the concerns of Journal A to Journal A during this time frame of 4 months? From my point of view, as the paper is not formally submitted to journal B (it won't be publish if I do not send a revised manuscript), I can use these 4 months to send out a revised manuscript to Journal A and get their decision by the end (hopefully) of 4 months. I don't think declining to revise a manuscript in journal B would be unethical, but since this is not so common situation, I would really appreciate if you have any informations.