There are three ways:
Lean a ladder against the lower wall, put boards connecting it with an upper stair, and put a ladder on top of that.
Use a baker scaffold. Baker scaffolds can be set up with varying height legs. Then put a ladder on the baker scaffold. This is my recommended solution.
Use a 2 wheel edging paint pad that will allow you to cut-in using a pole. http://www.decoratingdirect.co.uk/viewprod/f/FDDEDGPD/
If you are really careful, this might even look ok.