An apparatus for fluid catalytic cracking of a hydrocarbon feedstock, the apparatus comprising: a first reactor, wherein the first reactor is an up-flow dilute riser terminating in a dense bed portion at the top for the catalytic cracking of heavy hydrocarbon feedstock and is maintained under a first set of operating conditions, wherein the first reactor generates a first spent catalyst; a second reactor, wherein the second reactor is an up-flow dense bed riser reactor for the catalytic cracking of light hydrocarbon feedstock and is maintained under a second set of operating conditions, wherein the second reactor generates a second spent catalyst; and a single regenerator assembly coupled to, and shared by, the first reactor and the second reactor, wherein the regenerator assembly comprises: a regenerator vessel comprising a vertical internal baffle partition to divide the regenerator vessel into a first subunit and a second subunit, wherein the partition allows for overflow of excess spent catalyst from one subunit to another; a plurality of regenerator inlets in the regenerator vessel for receiving the first spent catalyst and the second spent catalyst by the first subunit and the second subunit, respectively, for regenerating the first spent catalyst and the second spent catalyst; an air distributor within the regenerator vessel for supply of air to the first subunit and the second subunit; an air controller to control air flow to the air distributor to generate a fully regenerated catalyst and a partially regenerated catalyst in the first subunit and the second subunit, respectively; a first regenerator outlet in the regenerator vessel for directing the fully regenerated catalyst to the first reactor and a second regenerator outlet in the regenerator vessel for directing the partially regenerated catalyst to the second reactor, for the catalytic cracking of hydrocarbon feed; and cyclones (21, 22) housed in the regenerator vessel to separate spent catalyst from entrained flue gases of the first and second subunits and remove the flue gases from the regenerator assembly. a first reactor, wherein the first reactor is an up-flow dilute riser terminating in a dense bed portion at the top for the catalytic cracking of heavy hydrocarbon feedstock and is maintained under a first set of operating conditions, wherein the first reactor generates a first spent catalyst; a second reactor, wherein the second reactor is an up-flow dense bed riser reactor for the catalytic cracking of light hydrocarbon feedstock and is maintained under a second set of operating conditions, wherein the second reactor generates a second spent catalyst; and a single regenerator assembly coupled to, and shared by, the first reactor and the second reactor, wherein the regenerator assembly comprises: a regenerator vessel comprising a vertical internal baffle partition to divide the regenerator vessel into a first subunit and a second subunit, wherein the partition allows for overflow of excess spent catalyst from one subunit to another; a plurality of regenerator inlets in the regenerator vessel for receiving the first spent catalyst and the second spent catalyst by the first subunit and the second subunit, respectively, for regenerating the first spent catalyst and the second spent catalyst; an air distributor within the regenerator vessel for supply of air to the first subunit and the second subunit; an air controller to control air flow to the air distributor to generate a fully regenerated catalyst and a partially regenerated catalyst in the first subunit and the second subunit, respectively; a first regenerator outlet in the regenerator vessel for directing the fully regenerated catalyst to the first reactor and a second regenerator outlet in the regenerator vessel for directing the partially regenerated catalyst to the second reactor, for the catalytic cracking of hydrocarbon feed; and cyclones (21, 22) housed in the regenerator vessel to separate spent catalyst from entrained flue gases of the first and second subunits and remove the flue gases from the regenerator assembly. a regenerator vessel comprising a vertical internal baffle partition to divide the regenerator vessel into a first subunit and a second subunit, wherein the partition allows for overflow of excess spent catalyst from one subunit to another; a plurality of regenerator inlets in the regenerator vessel for receiving the first spent catalyst and the second spent catalyst by the first subunit and the second subunit, respectively, for regenerating the first spent catalyst and the second spent catalyst; an air distributor within the regenerator vessel for supply of air to the first subunit and the second subunit; an air controller to control air flow to the air distributor to generate a fully regenerated catalyst and a partially regenerated catalyst in the first subunit and the second subunit, respectively; a first regenerator outlet in the regenerator vessel for directing the fully regenerated catalyst to the first reactor and a second regenerator outlet in the regenerator vessel for directing the partially regenerated catalyst to the second reactor, for the catalytic cracking of hydrocarbon feed; and cyclones (21, 22) housed in the regenerator vessel to separate spent catalyst from entrained flue gases of the first and second subunits and remove the flue gases from the regenerator assembly.