can two plainitffs' claims be severed if one dismisses out for not being able to bear litigation costs? (California)

Is this a case filed in California? In California, if a plaintiff dismisses their claims, then the defendant is the prevailing party, and if there's a contract providing for attorney's fees, this will result in a motion from the defendant to the talent plaintiff to pay the defendant's legal fees. If there's no contract with an attorney's fees clause, then the defendant can claim costs but not fees.

If this talent plainitff has no actual claims and is just a witness in the suit,. why is he a plaintiff?

Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

You can try but if the other plaintiff was included in your Complaint when it was first filed, it is not likely to happen unless the other plaintiff asks to get out of the case. Still, if you can show that his claims are substantially different from your claims, and that having his case tried at the same time as your case would be confusing, you may be able to get his claims severed from your claims at trial. If you already have an attorney, you should be discussing this with your attorney. He/she probably knows the best thing to do.