so i assume that before, you were having only 1 band, the GSM 900.
And now, you added a second layer of cells, in DCS1800.
You will have to optimize the parameters that control HO and Reselection between GSM and DCS. That is something that is dificult to do and that requires perfect knowledge of algorithms in your system. ndeed, those algo are very “proprietary”.
If you were using ALU I could help you.