The Collaborative International Dictionary of English v.0.48:

Coeternal \Co`e*ter"nal\, a.
   Equally eternal. -- Co`e*ter"nal*ly, adv.
   [1913 Webster]

         Hail, holy Light, offspring of Heaven first born!
         Or of the Eternal coeternal beam.        --Milton.
   [1913 Webster]