Q: Can we submit to another journal while one journal is waiting for a revised manuscript?

My manuscript was sent to a Journal A and was reviewed and finally rejected but with encouragements 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 immediately submitted the article to a Journal B. While Journal B was reviewing, we were able to address the concerns of Journal A as they are relevant for the study. We were also able to address the tricky experiment. Then, Journal B sent us back a notification that the paper is interesting, and gave 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 addresses 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 published 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 a not so common situation, I would really appreciate if you have any recommendations.