Your 9GG disk is a [Single-Ended SCSI-1 device|http://www.sunshack.org/data/sh/2.1/infoserver.central/data/syshbk/Devices/Disk/DISK_Fuj_MAB3091SC.html] with an 80pin SCA connector.
Your 18GB disk is a [LVD SCSI-3 device|http://www.sunshack.org/data/sh/2.1/infoserver.central/data/syshbk/Devices/Disk/DISK_Fuj_MAN3184MC.html] with an 80pin SCA connector.
Those SCSI technologies are usually incompatible. The LVD drive will probably just be an inert lump that isn't recognized.
If you do get anything to work, you'll just be adding 9GB of storage capacity -- there will not be any increase in speed because that is managed by the SCSI controller that is integrated on the Ultra-2's systemboard (which is Single-Ended SCSI-1).
Again, you won't damage anything by merely trying.