A catalytic cracking process is provided which comprises:(a) cracking a first heavy hydrocarbon feed in a first riser in the presence of a mixed catalyst composition comprising, as a first catalyst component, an amorphous cracking catalyst and/or a large pore crystalline cracking catalyst and, as a second catalyst component, a shape selective medium pore crystalline silicate zeolite, to provide gasoline boiling range material and one or more light hydrocarbons; and,(b) cracking a thermally treated second heavy hydrocarbon feed in a second riser in the presence of said mixed catalyst composition and in admixture with a gasiform material contributing mobile hydrogen species and/or carbon-hydrogen fragments at the reaction conditions employed to provide gasoline boiling range material in increased yield and/or of higher quality.