your 1st sentence is confusing. “bcch periphery is more than tCH”. What does it mean ?
BSC will allocate the MS on a 1800 TCH based on rxlev of 1800, traffic_load of 900 and traffic_load of 1800.
Timing advance is usually not taken into account for multiband networks.
Other parameters include “time” during which the MS is around a 1800 cell. If too short, then Ms stays in 900. If longer than a threshold, then it is handover to 1800 cell.