this is how i do:
1)use vertex tool to manipulate vertices
http://imgur.com/r1wGeWO
http://imgur.com/Harw3Ym
2)use clipping tool to slice brushes, you can change clipp modes by clicking on the Clipping Tool icon, I use when both parts stays white, after sliceing i delete parts that i dont need
http://imgur.com/pfuwCLY
http://imgur.com/akhLi4C