Source : Webster's Revised Unabridged Dictionary (1913)

Copartner \Co*part"ner\, n.
   One who is jointly concerned with one or more persons in
   business, etc.; a partner; an associate; a partaker; a
   sharer.

         the associates and copartners of our loss. --Milton.

Source : WordNet®

copartner
     n : a joint partner (as in a business enterprise)