PhD supervisor is not willing to co-supervise?
I think @BobBrown summarized everything that needs to be said.


  Brown's Rule for finishing the Ph.D.: Listen closely to what your
  supervisor tells you to do and do it. You can do the other research
  later.


In detail:


You have some publications from your cooperation with your main supervisor. Excellent.
Publish another paper with your new collaborator, if you have not done already. 
Then do another paper (those minor results) with your advisor. Finish everything there is to be done on this direction fast.
Finish PhD. 
Do whatever you want afterwards.


This is by far the easiest route to your PhD.