Is it okay to submit a modified journal paper that is still under review to a conference? I have a paper submitted to a journal that is currently under review.

My research supervisor now wants to then submit a similar paper to an upcoming conference, by taking an older version of this same paper, and removing some content to fit the conference page limits.

I would understand the conference paper to make sense for a work-in-progress, to have been submitted at an earlier time before a submission to a journal for review, but the work has already been completed.

Would there a possible ethical problem with submitting a purposefully stripped-down version of a work to a conference, after a journal submission had already made?
From two responses in the comments, submitting to a conference after a submission to  a journal would likely have policy violations from both the journal and the conference. 

(#1)


  You would probably violate the journal's policy. Usually, you are
  required to declare that the work has not been submitted/published
  anywhere else. If it has, then you need to note the differences.


(#2)


  you might also violate the conference's policy: The same rules apply.