[58224]

Beyerdynamic DT770

 

[PleasantSounds]

What  may work better is something like this:
 
http://www.amazon.com/Speaker-Headset-Switching-Manhattan-172851/dp/B000K8PH8C/ref=sr_1_19?s=electronics&ie=UTF8&qid=1357523561&sr=1-19&keywords=stereo+output+switch
 
Note that I'm not recommending this particular device as I have no experience with it - just wanted to give you an idea of the type of device you need.
 
An alternative is a simple extension cable that would let you switch between headphones and speakers without leaving your chair.
 
If you tried to feed two line level inputs from the splitter, that most likely would be fine. But having your headphones plugged in while listening to speakers would have a significant impact, as headphones have much lower input impedance and would "steal" most of the signal. Even worse: the headphone impedance tends to vary with the frequency, and that would surely impact the frequency response of your speakers.