My boot drive is an SSD drive which is great for speed and launching apps. SSD's aren't so good for hosting your home dir. So I've got it on a normal HD.
I tried the --add-group-disk option and it worked. It mounts both drives before booting the OS. So I don't see how I'll get data corruption or unbootable installs? Both drives are part of the same drive group with my boot drive being the base disk, both are instrumented by the bootstrap login.
I also have an emergency user whose home folder is on my boot drive so if I need to perform some kind of decrypt on my 2nd hard drive, I can do so without booting to a USB drive.
I was able to acheive this setup by doing the following:
1. Encrypt my boot drive normally. When this is complete go to the next step.
2. pgpwde --add-group-disk --base-disk (put your boot disk #) --disk (put the 2nd disk #) --auto --passphrase 'password' --user 'your name'
This should cause the 2nd disk to start encrypting. When this is finished, you are done.
Now when you boot up, both drives will be unlocked before the OS loads. I actually added a third external disk (just in case it was plugged in.)