How to decline an invitation to work on a funding proposal? Prof. A and I were working on a proposal which we intended to submit to a current call. Upon learning from my ex-supervisor Prof. B that she was also preparing a similar proposal to submit to the same call, I told Prof. A about it, and he suggested that we submit to another call, whose deadline is about two months later. Recently, however, Prof. B invited me to join her team for the current call. 

I am considering the following scenarios:


I decline Prof. B's invitation. What should I do in this case? I think I should let her know that I have been working with Prof. A on a similar proposal but planning to submit to a future call. Is this the right thing to do? How do I decline Prof. B's invitation without disappointing her? 
I accept Prof. B's invitation and do not continue my plan with Prof. A. Under what circumstances can this be justified? 
I join both teams. Under what circumstances can this be justified?
Whatever you do, both Professors A and B will certainly find out.  So why not just ask them now how they feel about this dilemma?  Perhaps you could even get them into the same room for a discussion.  You obviously like them both and would like to work with both.  Now you're simply trying to do right by both of them.  Who knows.  Unless A and B don't like each other, maybe there's a fourth option where all 3 of you work together.