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). We do also stock the O2NE+ and Safe-Ox+ which also have a 4-20mA output option which will output O2 information.