might I suggest downloading Line In it's free and it allows you to do what you are asking for. Set sound flower 2nd chnl to your headphones then do this with line in.
Then once you have done this go into System Preferences of your Mac.
Click Sound Settings and in Ouput select Soundflower (2ch) like so
What this does it takes all the desktop audio such as youtube or iTunes and it transfers it into your headphones so that you too will be able to hear it. Hope this helps.