No, you cannot submit several manuscripts to the same conference or publication. You must receive negative comments before submitting it to another location. You may even devise a backup plan by submitting pre-submission enquiries to many journals at the same time. But otherwise, yes, you should create a unique manuscript for each presentation or publication. The only exception is if you are presenting results from related work that was not included in your main study. In this case, you can use the same abstract and key words for both presentations. The reviewers will be able to connect the papers if they review them together. However, it is not recommended because the reviewing process is not guaranteed to be fair. If you do this, you risk being rejected by all but one of the venues you send to. This doesn't mean you shouldn't try it out, but you should be aware of the risks.