..........why?
the purpose of using headphones is to limit the amount of ambient noise you make so that you do not disturb others. with speakers playing there isnt a point to using headphones.
if you have regular speakers playing you can already hear sound so why would you want to put headphones on your head?
normally when you plug in headphones the default audio device switches to them. this is why the sound shuts off on your speakers. you could manually go back in and change the default audio device but i'm not sure you can run both with those settings alone.
a splitter cable on the speaker out ports would most likely work but again, there isnt much point to it.