quite simple,you just need to split the speaker input into two,here's a simple diagram I whipped up ;
![](https://acidmods.com/forum/proxy.php?request=http%3A%2F%2Fi45.tinypic.com%2F14bjakw.png&hash=8b1744276c8517ed869c29787ae16231ae0b386f)
The values of the potentiometers,transistors,boost voltage etc. vary on the operating voltage of the headphones you're using,you'll have to work the out for yourself,but all in all it's quite a simple project.
Hope I helped,
Ryan.