The simplest schematic for a headphone splitter is to get three stereo jacks, or two and one stereo 1/8" plug. You connect all grounds toguether, all left channels toguether, and all right channels toguether. That'll split your computer output into two.
Now, if you want to amplify your signal and split it, instead of just splitting it, building an amp is quite easy. I noticed that you put a pic of the boostaroo, which is probably the worst amp you could ever buy. You can build yourself a simple and much much better amp with the opa2132/opa2134 and dozens of others.
**broken link removed**
Thats the schematic you would use.
But if you just wanted a splitter, the first suggestion works perfect.
-Ivan C.