Are you with a Co-operative Housing Society in Maharashtra? If so the names of A and B would have been entered in their register, one as member and the other as Associate member. Only the main member can give nominations. If A was the main member, on his death the Society can transfer the flats to his nominees without asking any further questions. Even B will cease to be an Associate Member. Under law a nominee will not become an owner on transfer of the flat to his name. He will be only holding the property as a trustee. He has to transfer the property to the legal heirs or the successors according any will on demand. If no demand is made, it will become his property with lapse of time.
If B has any beneficial interest in the property he must register the same with the Society with necessary evidence. For instance B will have beneficial interest in the following cases among others
1. If the transfer deed for the purchase of the flat contains his name also. If his share had been specified in the document, his claim will be in accordance with that. Otherwise it will be deemed to be 50%. But if B keeps quiet, he will lose his claims, if any.
2. If both A and B had inherited the property from a common ancestor.
According to the Maharashtra Co-operative Societies Act, 1960 an Associate Member will have no right with regard to his membership except to vote at General Meetings in the absence of the main member.
It would be advisable for B to make his rights and claims to the Society during the lifetime of A itself to avoid disputes later.