The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         the associates and copartners of our loss. --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

copartner
    n 1: a joint partner (as in a business enterprise)

Bouvier's Law Dictionary, Revised 6th Ed (1856):

COPARTNER. One who is a partner with one or more other persons; a member of
a partnership.