How do I tell my teacher I want him to be co-advisor but not advisor? 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.
I don't know if there is some kind of recipe for doing that.

I think that the main point in your question is about the feelings of the involved advisor and co-advisor. If they are as professional as I suspect, then you just need to explain to both advisor and co-advisor the situation, and probably they will agree with your decision, or better, they can point out things that you haven't see yet.

I think that the key point in this process is avoiding the feeling of being "betrayed". This could be achieved being transparent with both parts. Maybe you must appoint a meeting with both advisors.

Finally, I'm not aware of the actual rules of the Universities A and B, but in the case I know (Universidad de Buenos Aires) the advisor and co-advisor have the same level of responsibilities, being the "co-" thing because you need more than one "mentor". In such case the co- thing is a trivial issue that could be easily agreed by the parts