I am currently a master student at university A after having done my bachelor at university B. The class sizes are small in this field, so I am quite close to teachers at both universities. I want my old teacher at university B to be my advisor in my future PhD, because the research I intend to do fits slightly better with his own work, and he will be much better at advising me with other directions of research (based on my experiences so far). However, my current teacher has a lot of knowledge in some subfield which will be relevant for at least part of the research project, so my intended advisor and I would like him to be participating as co-advisor.
How do I tell my current teacher I want him to be co-advisor? I think he expects to be my main advisor and obviously want to remain on good terms.
Note: I have seen How do I tell my current research advisor I want to work in a related field with another professor?, but I feel this is a more complicated situation since I still want my current teacher to participate in the project.