I'd like to ask two professors who work at my university to be the co-advisors of my Masters thesis (mathematics).
They work in the same research group on (almost) the same topic (which is well-aligned to my interests), although they actually never published together, and get along well.
The first one (A) is an Emeritus professor who has been for over 40 years one of the world leaders in my research area and is still very active; the second one (B) is younger (mid-forties) tenured professor but has already established a very good research reputation; also, he is A's former student.
In my opinion they complement each other very well and working with the two of them will be an extremely instructive experience.
Who should I contact first to talk about my research proposal to avoid being disrespectful to either of them?