Whatever you do, both Professors A and B will certainly find out. So why not just ask them now how they feel about these options? 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.

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.