1. From 900 cell to 1800 cell same site:- You can activate DADLB(Direct access to desire layer/band) feature.
Check the value of NBL(if less then more traffic would transfer on 1800) for 1800 segment.
Check LAR and LER PARAMETER as well.
2.from 900 of congested cell of site to 900 less congested cell of other site :-
Enable direct retry,TRHO.
If lots of handover call then you can change also the BLT value of congested site as well.