A special tool in Hammer that allows you to cut a brush into two parts. You can choose to save either part of the brush that it has created, or save the lot and move them individually. Clicking the tool icon will switch between parts to save.
Use with caution! it can cause compiling problems. That said, give it a go and try it out but you might want to read Atom's
tutorial on Vertex Manipulation first. It a safer way to make those shapes you want.