Bierzo,
There are several ways you can do this. You can specify your channels as a comma seperated list (ie. Dev1/ai1,Dev1/ai7). You can also call DAQmx Create Channel twice (once for each of the channels you want to read), and use the same task handle in both instances. Either of these methods should allow you to read only the two channels you are interested in.
I hope this helps,
Dan