A gas delivery system for a substrate processing system includes a first manifold and a second manifold. A gas delivery sub-system selectively delivers gases from gas sources. The gas delivery sub-system delivers a first gas mixture to the first manifold and a second gas mixture. A gas splitter includes an inlet in fluid communication with an outlet of the second manifold, a first outlet in fluid communication with an outlet of the first manifold, and a second outlet. The gas splitter splits the second gas mixture into a first portion at a first flow rate that is output to the first outlet and a second portion at a second flow rate that is output to the second outlet. First and second zones of the substrate processing system are in fluid communication with the first and second outlets of the gas splitter, respectively.