Inner/Outer (Inside/Outside) margin in Pages
I think it was fixed in newer version of Pages. When I asked this question more than 2 years ago I switched to Word because I couldn't make this work.

Solution in Pages 10.0 (6748):

Go to Document Settings and under Document Margins select Facing Pages. Now Left/Right margin will change to Inside/Outside. Result of setting inside to 2cm and outside to 5cm: