Is it considered self-plagiarism if I submit a part of my own paper as another 2-page demo paper for demonstration purposes?
As long as the demo paper is not peer-reviewed and not published in any kind of proceedings, this should not count as self-plagiarism because it does not count as a proper publication. I would put it more in the same section as giving a talk about your results. 

You should however check the regulations of the conference you originally submitted to. The author guidelines normally have section about dual submissions that clearly define if, for example, an arxiv submission is allowed and what you should or should not do to maintain your anonymity. 

If the conference allows to publish a preprint on arxiv, you can upload it there and cite it in your demo paper.