what to do over summer

Personally, I think 2 research internships is too much, but I have known people who have done 2+ research internships at the same time while in school so that is a matter of your work ethic. Yeah, it is realistic I suppose since you should have something to do over the summer. It will be a waste otherwise. Nonprofit organizations are usually hard to work in unless they have a application window so cold emailing them might be difficult but you might get lucky.

For tips, if you want to get into research, you can always look for research programs like MOSTEC at MIT or you can cold email professors in your local area. More often than not, you are going to be rejected or ignored but sometimes, if you show enough enthusiasm and experience in the subject that you want to research about, the professor may let you join in their research. I don't know what nonprofit organizations are around you so I can't really help but you can start your own nonprofit organization with a group of friends. That way, you don't need to worry about getting into a nonprofit organization while showing you have initiative and leadership capabilities while demonstrating your interest in helping others.

I hope this helped, but if it didn't then hopefully someone else provides a better answer.