Conversion of single flat into two flats during redevelopment of building

Our co op hsg soc situated in mumbai suburbs is under redevelopment process voluntarily. Terms with existing flat owners are finalised. Presently flat area is 600 sq ft carpet. Builder is giving certain additional area free of cost (25 %) and offering concessional rate for additional purchase area upto certain sqft. Few society flat owners want to convert their single flat into two different flats as they are getting additional free area and can buy certain additional area at concessional rates. This makes sense for them to make two 400 sq ft carpet (appx) flats after redevelopment. Builder has agreed to prepare two separate units out of such one unit to all flat owners who wish the same. However, we need to know whether this is leagally possible and what procedures are to be following by society as well as builder in order to register the flats as two separate units in govt. records and also in society records after redevelopment. The original purchase agreements and share certificates of single flats are either in single name or in joint names. Please guide...