Abstract: A gas delivery system for substrate processing tool includes a first gas box configured to supply a first gas mixture including one or more gases selected from a first set of N gases to a first substrate processing chamber, where N is an integer greater than one. A second gas box is configured to selectively supply a second gas mixture including one or more gases selected from a second set of M gases to a second substrate processing chamber, where M is an integer greater than one. A third gas box is configured to supply a third gas to the first substrate processing chamber at a first concentration and to supply the third gas to the second substrate processing chamber at a second concentration. The third gas is incompatible with one or more gases in the first set of N gases and with one or more gas in the second set of M gases.