VisualSVN Server does not support such configuration, you ether have to use Windows or Subversion authentication.
There is a workaround however: you may have 2 VisualSVN Servers which
work with the same repositories root folder. This way you are be able
to enable Subversion authentication on one server and Windows
authentication (Active Directory) on another.
However this is not the best solution since you'll have to maintain two separate access permissions lists: for Active Directory users and for Subversion user accounts.