it is a very very very complex optimization.
Between these two cells, use the preferred band HO from 900 to 1800, and disable (or slow down) pbgt HO frfrom 1800 to 900.
EN PREFERRED BAND HO = enable
MULTIBAND TRAFFIC CONDITION = any
L RXLEV CPT HO(900,1800) = -75dBm
EN MULTIBAND PBGT HO = disable
HO MARGIN (1800, 900) = +20dB
EN_TRAFFIC_HO(1800, 900) and (900,1800) = disable****
HIGH TRAFFIC LOAD = 85%
A TRAFFIC LOAD a bit less than default
Then you fine tune the L RXLEV CPT HO(900,1800) = -75dBm
-73dBm to reduce load in 1800
-77dBm to increase load in 1800
Also, check that Cell reselect offest of 1800 is not too high (CRO = 4dB or 6dB is enough, usually)