Is it possible to use USB output and audio port output at the same time? I want to use two headphones simultaneously; one USB and another through the audio port listening to the same audio source (i.e iTunes, Spotify, or Netflix) at the same time. Is it possible? 

Info about my device, 

MacBook Pro (Retina, 13-inch, Early 2015)  
2.9 GHz Intel Core i5  
OSX El Capitan 10.11.6
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.