How do I tell my teacher I want him to be co-advisor but not advisor?
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