The difference in power between 1800 and 900 cells is usually like 10dB, so if u got like a twin layer, trafic will normally be on 900 cells. What you do is:
-favour intialisation on the 1800 cells, e.g. putting a Cell Reselect offset on them (14, 16, 20, 24, 30, etc, depending on the % of trafic you want on this layer).
-Maintain the trafic on the 1800 cells and favour HO towards them: increase HO_margin from 1800 to 900 cells, and decrease from 900 to 1800 cells.