The following ways are possible:
1.) Using RAID1 Image:
The 2nd HDD is used as the mirror of the 1st HDD, thus providing moredata safety.
2.) Using RAID0 Image:
The 1st and 2nd HDD are interconnected (so the size amounts to 1st + 2nd HDD)
Drawback: One failing HDD causes ALL data to be lost irreversibly.
3.) Create filesystem manually and mount into the system:
An example (WARNING: That does NOT pertain to all images with the label "RAID"!):
Only the 1st HDD is used with minimal installations (or normal PLESK installations without RAID). The 2nd one can be mounted under any desired mounting point after you have created an according filesystem.
You can do this via SSH as follows, for instance:
Log in via Putty.exe (User = root and the default password).
Putty is an SSH client for Windows and available for free, see LINK
* enter fdisk /dev/sdb
* create a new primary partition
* create a filesystem via mkfs -j /dev/sdb1
* create a mounting point: mkdir /data2
* mount it via mount /dev/sdb1 /data2
Now the filesystem is available on the 2nd HDD under /data2.
This has to be entered into the file /etc/fstab in order to remain valid after a reboot. The file is usually documented well.