Is it OK to use the content of my own paper which is not yet submitted, in my other paper?
Most journals require that the submitted manuscript has not been submitted total or partially to any other journal. Of course different policies apply to open access journals. If I were you I would check the journal's policy regarding this issue and contact the editors if necessary to get their opinion (probably they will require details on what will be duplicated). To the very least discuss this with your supervisor. You can find yourself in a very unpleasant position if you are not careful (not to mention having your papers rejected), as indicated by @Enthusiastic Student