The capability in SCCM to auto-enroll a client to MDM is part of the
onboarding experience to connect the site with Intune. So yes you do
need to configure co-management for the automated enrollment. However I
think you can manually MDM enroll.
Which one wins (ConfigMgr or Intune MDM) if you manually enroll ConfigMgr client to Intune without co-management and you e.g. set different update settings in both tools? Last writer wins? Because Windows 10 1709 allows to have both mgmt tools at the same time, there is a strong probability that multiple devices will have these issues by accident.