If I fit the AX50 (with O2) and a 4-20 mA output, does the 4-20 mA send information about both sensors or only one? 



The Ax 50 is primarily a CO2 detector, therefore the 4-20mA line only outputs the CO2 information (not O2). Our newer Ax60+ with the optional data output module can output 4-20mA or MODBUS from each sensor separately, the system can comprise up to four sensors which in any combination of CO2 or O2 sensors.