A simplified example. This may not be the best solution but it is tested and working.

2 physical disks with partitions

• /dev/sda1 (/boot)
• /dev/sda2 (raid volume)
• /dev/sdb1 (/boot.sdb):
• /dev/sdb2 (raid volume)
• /dev/md0 (LVM physical volume)
• /dev/mapper/VolGroup-lv-swap etc..

Command line

cp -a /boot/* /boot.sdb/
umount /boot 
umount /boot.sdb
mount /dev/sdb1 /boot
grub
      device (hd0) /dev/sdb
      root (hd0,0)
      setup (hd0)
      quit

Reboot, change BIOS boot to sdb and enjoy.