What platform are you using? On a Sparc platform, you have to install the boot block on the root partition. So if you install one file system on /dev/rdsk/c0t0d0s0 and another one on /dev/rdsk/c0t0d0s6, you will install the boot block on both partition 0 and 6. To select which filesystem you boot your workstation to, you would use:
boot disk0:a "For Partition c0t0d0s0"
boot disk0:f "For Partition c0t0d0s6"
To install the boot block, change directory to: /usr/platform/<workstation platform>/lib/fs/ufs
/usr/sbin/installboot ./bootblk /dev/rdsk/c0t0d0s0 "for partition (a)"
/usr/sbin/installboot ./bootblk /dev/rdsk/c0t0d0s6 "for partition (f)