Hello Irene. When acquiring more than 1 channel, you should still only use one DAQ Assistant. If, for example, you have 2 analog inputs that you are trying to acquire, and you place one DAQ Assistant for each analog input channel, you will get an error because only one DAQ Assistant can access AI channels at a time.
You can add channels to your DAQ Assistant by clicking on a plus sign by the "Channel List" in your DAQ Assistant. Without knowing what version of DAQmx you have, I cannot be much more specific since the location and color of the plus sign will vary from version to version.
However, if you refer to the website below, and look at the picture of the "DAQ Assistant" window early in the article, you will see a green plus sign directly under the words "Channel List." If you click on this, you will get an option to configure another channel.
Learn 10 Functions in NI-DAQmx and Handle 80 Percent of Your Data Acquisition Applications