Need suggestion on simultaneous submission in journal and conference
It sounds like you've gone in the wrong order, here.  In computer science, it is common practice to initially publish a work in a conference, then the follow it by an extended version in a journal, which is more thorough and takes into account all of the feedback from the conference and advances during the time since the conference version was completed.

Going the other way, however, strikes me as unethical and a form of self-plagiarism.  You have already produced a journal-scale paper, and mining a smaller conference paper out of it is just double-publication.  Now, in certain cases a conference might accept this if explicitly declared (there are some conferences that have a special track for reports on journal publications).  If the conference expects original work, however, you cannot ethically submit your paper to the conference.