Legal Question in Real Estate Law in California

Yes

Well, yes and no. Each co-owner can sell his share without the signature of the other. It would take the cooperation of both to sell a 100% interest. There isn't much market for half interests, so such sales are pretty rare,

If the two co-owners happen to be married to each other, the rules are a little different, but the result is both have to participate in the deal for ownership to transfer in most circumstances.

It depends on what capacities these two names hold as stated in the title deeds.

If both names are described as owners and in absence of respective share of ownership, it is deemed that both have equal but undivided share.

So sale must be signed by both names for 100% ownership transfer, otherwise each name can transfer only 50% undivided share in property.