Is it considered self-plagiarism if I submit a part of my own paper as another 2-page demo paper for demonstration purposes? Recently I submitted an IEEE conference paper about an open-source machine learning project/framework related to mobile networks. Now, I also want to demonstrate its features at another IEEE conference using a 2-page demo paper.

Considering the original paper is still in reviewing process, is it ethical to submit a related demonstration paper with the same approach to another conference. I mean from self-plagiarism aspects, can I use the same title and experiment results in demo paper as well just for showcasing purposes? If not, how can I also reference the original paper considering it is not published/accepted yet!
As long as the demo paper is not peer-reviewed and not published in any kind of proceedings, this should not count as self-plagiarism because it does not count as a proper publication.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.