Question about fstab

[Дмитрий Бехтерев]

Hello all!
I want use redundant scheme for booting my OS. For instance I have two ufs
slices and each of them keep /boot folder. For example, I want use fstab
like that:
# Device                Mountpoint      FStype  Options         Dump
Pass#
/dev/ad6s1b             none            swap    sw              0       0
/dev/ad6s1a             /bootdir        ufs     rw              1       1
/dev/ad4s1a            /mnt/bootdir    ufs     rw              1       1
That scheme will work unless one of ufs slices go out. After that I just
select bootable disk in BIOS and reboot. But during boot process I have got
error that one of slices can't be mounted. It's not convinient. How can I
get decision?
Thank all.