The Collaborative International Dictionary of English v.0.48:

Coexistent \Co`ex*ist"ent\, a.
   Existing at the same time with another. -- n. That which
   coexists with another.
   [1913 Webster]

         The law of coexistent vibrations.        --Whewell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coexistent
    adj 1: existing at the same time [syn: coexistent,
           coexisting]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "coexistent":
   accompanying, accordant, affirmative, agreeable, agreeing,
   answerable, at one, coetaneous, coeternal, coeval, coexisting,
   coherent, coincident, coinciding, coinstantaneous, collateral,
   commensurate, compatible, concomitant, concordant, concurrent,
   concurring, conformable, congenial, congruent, congruous,
   consentaneous, consentient, consistent, consonant, contemporaneous,
   contemporary, conterminous, cooperating, cooperative,
   correspondent, corresponding, coterminous, en rapport, equivalent,
   harmonious, in accord, in agreement, in rapport, in sync,
   in synchronization, inaccordance, inharmony, isochronal,
   isochronous, like-minded, of a piece, of like mind, of one mind,
   on all fours, positive, proportionate, reconcilable,
   self-consistent, simultaneous, symbiotic, synchronal, synchronic,
   synchronized, synchronous, unanimous, uniform, unison, unisonant,
   unisonous