See his VOD cast - http://www.justin.tv/eg_idra/b/278424082
I'm noticing he's executing some command while having all drones selected.
So, I looked as the link and here is what I saw:
Let's see it in slow mo:
Its in 2 seconds he has split his drones in groups of 2. Its beautiful, like watching ballet
Another way to split drones is to hit the hotkey to select a single idle drone, and click a mineral patch. Do this 6 times really fast, and each drone goes right to its own mineral patch. Something like,
This is much easier than selecting the individual drone and mineral patch, but it's still to hard for me to do effectively.
But honestly, as @Aardvark says, the initial split is not that important. This amount of analysis / practice would be better spent on something like hydra micro or macro.
So while I have seen this done in game, it's something I don't care to perfect.
You may be witnessing legendary micro but it is probably just a lag spike (where he is in the game and can select each unit individually). Most players do a 3/3 split to two sets of minerals.
To do this hold CTRL during the loading screen and press F1 when you enter the game (select all + idle workers) right click on one mineral node then box 3 drones (drag and select) and have them go to another set of minerals.
Keep in mind though that in the scheme of a game of starcraft your initial worker micro is pretty small potatoes compared to your decisions later in the game (forgetting to build workers, supply blocking etc)