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)