I installed Ubuntu 14.04 on 2x 3TB mdadm raid 1 and wanted both drives to be bootable in case one fails. This took a rather long time to figure out but this is how you do it. Below was tested on a vm.
GPT Partition Layout
3TB mdadm raid1 / lvm
1. Set labels on the /boot
2. Modify fstab to mount by label
3. Copy /boot to /boot of second drive
4. Update grub on the second drive