I've seen the recommendations of software raid0. The answers are all technically correct.
Don't do this. Especially with real hardware. If you lose one drive, you will lose both the drives.
Could you give us more info on how your other encrypted drive is mounted and meant to be used?
- It is possible to get the second encrypted drive to decrypt and mount automatically, without logging in.
- It's possible to do it without leaving a key for your second drive on your primary hard drive using a TPM
- It is possible to get both drives to decrypt automatically and mount
It all depends how the encryption is being done, and where the mount points are.
Could you give us:
- The rough dimensions of the drive partitions (fdisk -l)
- Your current mount points after mounting the second drive (df -h)