You can't split a brush into more then one brush, but you can get a similar effect by creating a new brush through the old brush, selecting the new brush and doinga csg subtract. However I HIGHLY RECOMMEND TO EVERYONE: NEVER USE CSG SUBTRACT. Just create a second brush.
Clipping in Radiant means, cutting pieces off of a brush. Think of 2 point clipping as cutting the brush with a saw, however all the pieces left over disappear (why it can't be used to split a brush).
To do it, select the brush, and press X. Then select cut points by clicking where you want (like drawing the line you will cut along). When you have 2 points selected, the shape will change in the grid to show you what the new brush will look like. If you are happy with it, press enter to accept. If not, press esc and try again.