How do I tell my professor that I am volunteering with another project? Should I? I am working on a project as an undergraduate researcher in Prof A's lab. 

At the end of the semester, one of my class professors, Prof B, proposed an idea for a project, so I and a few other students volunteered to research with him. Since this is all over the summer (meaning no classes), the topic was interesting, and I have nothing to do due to the pandemic, I thought it was okay (time management isn't an issue). The work with Prof B isn't part of a dedicated lab either, and I expect it to last just a few months in the summer. 

However I came across this question (https://academia.stackexchange.com/a/47981), and now I am concerned that I am lying by omission. How do I let Prof A know about Prof B, or should I even worry about this? Was it wrong of me to volunteer with Prof B this summer? My question is different because I have no other obligations.

The last thing I want is to worry Prof A or make him unnecessarily think that the lab is boring, or for him to get a bad impression of me. Time management isn't a problem, and I've been attending &amp; contributing in all meetings and deadlines, etc.
If you can handle both projects without compromising either, then there should be no issue. There, actually, shouldn't be a reason not to tell the other professor and they should support you as long as you haven't made a commitment to full time work on either project. 

Taking on an additional project shouldn't give the impression that the earlier one is boring, just that  you want some additional experience. 

On the other hand, if you are working satisfactorily with A, there is no real need to complicate things and talk about the other. 

Think about the personalities of the two professors and choose. I don't see any ethical constraints either way, as long as you can do the required tasks.