So assuming 1st half of /(root) fs is on c0t0d0s0 and attached disk is c0t1d0s0.
I should now install boot block on second disk:
installboot /usr/platform/`uname -i`/lib/fs/ufs/bootblk /dev/rdsk/c0t1d0s0
and then set alternate disk to boot in case emergency. Like that:
setenv boot-device disk0 disk1
eeprom "boot-device=disk0 disk1"
Is that correct ?