Legal Question in Real Estate Law in California

The buyer can request specific instructions in the purchase and sale agreement as to how long the seller can occupy the property during escrow and after, but there is nothing the buyer can do if the seller does not comply.

Mr. Roach is basically correct. All you can do is contract for the seller's departure and sue for breach of contract and unlawful detainer if they do not comply. You can, however, include some contractual incentives for them to comply. For example, I assume you would be getting rent for the two months at something around market rate. You can provide that the rent,say, doubles if they do not vacate as agreed. You could also set up a reserve in escrow of some agreed amount as security for them leaving, and provide that if they do not vacate on time you get that money back. That would at least fund the eviction.