From Richard Winger at Ballot Access News:
On January 11, the American Independent Party asked the California Secretary of State to list these twelve candidates on its presidential primary ballot: Lawrence Beliz, Wiley Drake, Brian Henry, Tom Hoefling, Rex Kafo, J. R. Myers, Walter Niyakik, Robert Ornelas, George Peabody, Robert S. Peters, Robert Sisilo, and Donald Trump.
California law says presidential primary candidates mentioned in the news media should automatically be placed on ballots. Because the news media typically doesn’t cover minor party presidential primary nomination contests very well, if at all, the tradition in California and certain other states is that election officials simply ask the minor party which presidential candidates to list. However, that tradition was disrupted in California in 2012, when the former Secretary of State, Deborah Bowen, refused to follow the suggestions of the American Independent Party and the Peace & Freedom Party.
The AIP has information that Donald Trump is willing to be listed on the AIP primary ballot. The AIP lets independent voters vote in its presidential primary, but the Republican Party of California does not. The AIP says now independent voters will be able to choose an AIP ballot and express support for Trump in that manner.
As far as is known, no presidential candidate has run simultaneously in the presidential primary of two different parties in the same state. However, there is no law against it, if the candidate is willing. California and approximately 24 other states do permit two parties to jointly nominate the same presidential candidate in the general election. The last instances in California of two parties running the same presidential nominee in the general election were in 1940 and 1928.