What should I do when my supervisor wants to delay my graduation?
Based on your description, one of your supervisors is behaving in an unethical manner.  A faculty member must not delay a student's graduation purely for personal benefit.  Nor should they coerce a student into beginning a PhD under their supervision.  Most programs have a procedure for removing or replacing a supervisor.  I recommend you use it if your supervisor is behaving unethically.

You might get a better answer if you said why you have this supervisor or what your goals are.