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]