No, they would both have to agree (and sign). Right now, to oversimply, the property is owned 50-50; e.g. were it to be sold, A and B would each get 50% of the proceeds. If C is added and the property is sold, A would only get 1/3 or 33%; therefore adding C reduces A's interest by essentially 1/3. But B does not have any legal right or authority to take a away part of A's interest in property without A's consent.