How to handle getting offers from two PhD supervisors who know each other? The problem: I have a professor that accepted me to take on a thesis when i'm ready but another one offered me one also. The second one did it while the first professor was in the room and I felt the obligation to deny it, even though we have way better communication with the second one.

Both of them know each other very well (they even have neighboring offices) and denying one of them will complicate things and the worst case is that none of them will give me a thesis because I will seem too selfish.

The question:
How can I choose the second one (if the offer is still on) without making a mess?
I recently posted a question about being on the other side of this scenario - a professor in my university offered to take on a student who had already agreed to work with me (and had been working with me for the previous year already).

You'll notice from reading that question and its responses that while I was a bit annoyed with the professor, I wasn't upset with the student. In fact, I advised the student to choose the advisor he thought would be best for him, and gave him a good recommendation to the other professor in case that's what he chose. 

That's because the student is supposed to act in his/her own best interest (while still being responsible and professional, of course). It's not being selfish, it's being smart. 

The first professor shouldn't get upset with you for pursuing an opportunity that is better for you - if he/she does, then you really don't want to work with someone like that, anyways.

So, go talk to the second professor: "I was thinking some more about your offer to advise me on my thesis and I have reconsidered my original decision. Is the offer still available?"

If he/she says yes, accept the offer and go talk to the first professor: "I really appreciate your offer to advise me on my thesis, but I've decided to work with X instead. Thanks, again."