Legal Question in Intellectual Property in California

I don't think you are stating the law correctly, although I have an idea what principle of copyright law you have in mind. Under the 1909 version of the Copyright Act, there developed what is called the "indivisibility principle" under which courts held that there can be but one owner of a copyright, so that purported assignments of partial interests were merely nonexclusive licensees. In the 1976 Copyright Act, effective 1/1/1978, the indivisibility principle was generally abolished. See 17 U.S.C. 201(d) which permits a copyright owner to convey separately the exclusive rights created by copyright under 17 U.S.C. 106. Further, anyone holding an "exclusive license" created in writing may sue for infringement of that particular right. 17 U.S.C. 501(b).

Both the 1909 and 1976 Acts allow multiple individuals to be joint owners (or co-owners) of the same copyright. See House Report No. 94-1476 (1976), U.S.Code Cong. & Admin.News 1976, p. 5659 (�Under the bill, as under the present law, coowners of a copyright would be treated generally as tenants in common, with each coowner having an independent right to use or license the use of a work....�). This is probably the rule or principle that applies to your question, and it follows the broader principles of the rights of cotenancy ownership of property and is not unique to copyright law.

For a case showing how the 9th Circuit, covering California, might rule on questions of this sort, you might want to look up a case called Nafal v. Carter, decided in 2007 and published in the Federal Supplement case books at 540 F.Supp.2nd 1128.