Is it possible to use USB output and audio port output at the same time?
The short answer is YES.

What you are asking is called "aggregating your device."  This is done through MIDI Audio Setup found in Applications -> Utilities

In the image below, MIDI Audio Devices is opened up.  I have highlighed the two output audio devices I want to aggregate; notice both of them are outputs (0 in/ 2 out).



When you click the "+", select "Create Aggregate Device"



When you create your device, just select the devices from the right you want to aggregate into one.  In my example, I have selected the Built in Audio and my USB Codec from the previous example.



Close MIDI Audio Setup.  When you open Sound Preferences, you will see the new output device.  Select your new aggregated device to have sound output on all devices.