I am trying to use a single AM25T to measure both thermocouples and general voltage channels. In this case, is it possible to use one AM25T instruction? Or do I need to use two AM25T instructions, one for thermocouples and one for voltage measurements?
You will need to use two AM25T instructions because of the parameter called TCType. If you measured all sensors with one instruction and specified the type of thermocouple, then all of the sensor readings would be converted to temperature and you don't want that for your voltage measurements.
In one instruction you will specify the thermocouple type for measuring your thermocouples in teh TCType parameter. In the second instruction, you will specify -1 for TCType, which will provide an output of millivolts instead of degrees. For the second AM25T instruction, you will also need to specify for AM25TChan the channel number where your first voltage measurement sensor is connected.