That is possible, depending on your particular configuration of course. I can only help you based on my laptop (HP nx8220), which has only one headphone output. If I run KMix and go to the Switches tab, there is a Headphone Jack Sense option, which controls whether or not the speakers cut out with headphones plugged in. This is in KDE of course, I can't recall what the GNOME equivalent is, and you must have ALSA configured correctly to support your hardware. Good luck.