You have misinterpreted the concept hence you are furnishing confusing information in this forum while seeking opinion.
From your contents it can be understood that the property which belonged to X was orally partitioned between his children i.e. A and B.
Now A and B have decided to transfer their share in the property to C separately by executing separate registered gift deeds in his favor.
Since A and B are the absolute owners of their share in the properties, it is not necessary for their family members to sign the gift deed, however obtaining their signatures in the registered gift deeds will not in any way harm or cause legal hassle to the donee/beneficiary of the gift deed.
Upon this registered gift deed in favor of C executed by two brothers in respect of their respective shares in the property separately, C becomes an absolute owner of the property he acquired by these two different gift deeds with clear and marketable title to the property so acquired.
However you may have to obtain a proper legal opinion from a local lawyer before finalizing the deal for purchase of the said property to confirm about any other legal impediments/encumbrance that may subsist/exist
