In my department, every undergraduate (including me) has to do a Capstone project with a professor in order to graduate. Students need the professor's consent prior to declaring that the supervisor for the project is that professor. However, the rules do not indicate whether students can obtain multiple professors' consents and choose one of them as the supervisor (thus declining all others). I asked fellow students and they all talked with only one professor.
I talked to two professors and they both are happy to be my supervisor and now I have to choose only one of them. Declining a professor's "offer"  might be an issue to the professor because I suppose they have to make changes on their schedule (though the consent must be obtained two months before the project).
Both professors are fairly respectable and I do not want to be distrusted by them. What is the proper etiquette when declining one of them?
: They did not actually give a formal consent. They jumped directly into the discussion of interests and schedules, as if they have already treated me as their supervisee.