Dear Saikat Deb this is an important technical question which will certainly be of broad general interest to many other RG members as well. In general, there is nothing wrong or unethical about submitting two manuscripts to the same journal at the same time (even if the authors are the same). The key point here is that the articles must each contain a "full story" and should ideally cover different research topics. What must be strictly avoided is the so-called "salami-slicing", i.e. the undue fragmentation of research results. In this context, please have a look at the following useful link in which the pros and cons of submitting two manuscripts to the same journal at the ame time are discussed:

Submitting two or more papers to the same journal

https://www.editage.com/insights/submitting-two-or-more-papers-to-the-same-journal

My personal advice would be to better avoid such a situation. There will always be the risk that the editor suggests combination of the two manuscripts into one.

Good luck with your work and best wishes!

Dear Saikat Deb Saikat Deb, if the two manuscripts are on different topics, certainly you can. But if the manuscripts are on a similar topic, ypu risk the "salami-slicing" effect, as explained by Frank T. Edelmann Frank T. Edelmann. Thus, be careful ;-)

It cannot be considered "unethical" or "wrong" as long as the two MS deals with two separate topics. If the two separately contains part of findings for reasons better known to the corresponding author, it is better to send the second one to a different journal. It is my personal observations that reputed journals usually discourage publishing the articles from related set of authors in succeeding issues of the same journal.