Copartner is a joint partner, as in a business enterprise. S/he is a member of a partnership. The prefix "co" is a redundancy, since a partner is a member of a partnership.
The following is an example of a caselaw on copartners:
The legal title to partnership property is in the copartners, either of whom has authority to convey it in any transaction within the scope of the partnership. Each copartner has an unlimited power of disposition of his share of the partnership property, subject only to the claims of creditors of the firm, and of the other copartners on a settlement of the partnership affairs. If the firm owes no debts, and if there be nothing due from one of the copartners to the others, he clearly has the right, at his option, to sell and convey his interest in the partnership property, unless he be restrained from so doing by the terms of the copartnership. But the claims of the creditors of the firm, and of the other copartners, are only in the nature of an equitable lien on the partnership property, for the security, in the one case, of the demands of the creditors, and in the other, of the remaining copartners, for any balance found to be due to them on a settlement of the partnership affairs. This lien can only be asserted in a court of equity, which can wind up the partnership, marshal the assets, and ascertain the debts due from the firm, and the balance, if any, due to the copartners. A court of law is not competent to administer this form of redress, to adjust the affairs of the partnership and to ascertain and enforce the equitable liens of creditors and of the several copartners. [Stokes v. Stevens, 40 Cal. 391 (Cal. 1870)]