share declaration in property

Respected members, If a property is registered in name of two persons (case of joint property) and in registry their share is not mentioned then by default will it be considered as 50-50 share or not. Is it possible that they declare their shares legaly on paper at later stage. Say for example they want it to be 30-40 share how this can be done legally later on. One more thing can they include other members names in registry at some later stage legally. Thanks in advance Regards