Put the 1900 cells in the lower layer e.g. Layer 1 and 850 cells in the higher layer e.g. Layer 2. Then tune the parameter LAYERTHR (Ericsson) to shift the traffic between both cells. Brief description of the parameter is that upto which RXLEVEL the lower layer cell should be prioritized. For example if you have LAYERTHR=95 then this cell will be prioritized at any point where its RXLEVEL is greater than -95dBm whatever the RXLEVEL of higher layer cell may be. But when the RXLEVEL will go below -95 dBm, basic ranking will take over i.e. no prioritization.