The audioDeviceReader and audioDeviceWriter objects have "Driver" and "Device" properties. For example, if the output device supports 5.1 (6 channels), you could play this:
adw = audioDeviceWriter(48000,"Device","5.1 audio device");
for ii = 1:20
Note that pressing tab after "Device"," will bring up a list of devices in your system.
Same with the reader, except you specify the number of channels when creating it:
adr = audioDeviceReader(48e3,adw.BufferSize,"Device","Dual Microphone Device","NumChannels",2);
Calling adr() will return a buffer-long matrix with 2 columns.
Note that two different devices will have independant sampling clocks that differ ever so slightly (one slightly faster than the other), so if you use the writer and reader with different devices in a loop for a very long time without taking this into account, one device might underrun every once in a while (maybe after several minutes).