How do I tell my professor that I am volunteering with another project? Should I?
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.