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]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "coeternal":
   accompanying, ageless, agreeing, ceaseless, coetaneous, coeval,
   coexistent, coexisting, coinstantaneous, collateral, concomitant,
   concurrent, constant, contemporaneous, contemporary, conterminous,
   continual, continuous, coterminous, dateless, endless, eternal,
   eterne, ever-being, ever-durable, ever-during, everlasting,
   everliving, immemorial, incessant, indestructible, infinite,
   interminable, isochronal, isochronous, never-ceasing, never-ending,
   nonstop, nonterminating, nonterminous, olamic, perdurable,
   permanent, perpetual, sempiternal, simultaneous, steady, timeless,
   unceasing, unending, unintermitting, uninterrupted, unison,
   unisonous, unremitting, without end