Can two California Limited Liability Company enter into a simple general partnership for a one-time project?

yes, two LLCs can enter into a joint venture agreement.

Yes, this can be done. Most likely, you would want to start another LLC, for liability purposes, however.

You should bring on a good lawyer who knows film production law and business law. Most movies are a separate legal entity, or several separate legal entities for different portions of the film process. Generally speaking, a film project should never become entangled with any existing LLC, but should be its own entity, with its relations with other entities controlled by contract. Once a movie or a movie phase is complete, its entity should leave no ties to any other entity. The production entity may be separate from the post production and from the distribution. At the very least, each movie project should be its own entity. That is generally speaking and you need to get a lawyer and have them work on your situation in particular. The best thing any movie can do is to have good legal help all during the process. Legal work should be in any film budget.