1 earpiece = 1, 2 earpieces = 4

This is just an empirical measurement, but I'm quite sure that when listening to some music using only one earpiece the perceived volume is 1/4 than when using 2 -- not half.

I think this makes some sense, as the second channel adds one dimension to the sound, turning something mono-dimensional into bi-dimensional (the sound may come from anywhere on the plane where we stand).