I would like to add the second virtual hard disk to my existing virtual machine. How can I do that?
Start Parallels Desktop, do not launch your virtual machine.
Open the virtual machine's configuration > go to Hardware tab > click on + sign at the bottom > choose Hard Disk from the list.
In the appeared menu select Type: Existing image file.
Select its Location and find your second hard disk on the Mac.
When the second disk was found, choose it and click on Open button at the bottom > then OK.
The second hard disk will be added and automatically given the name Hard Disk 2 whereas the primary disk will be Hard Disk 1.
Make sure Hard Disk 2 is mapped to your virtual machine properly by checking it in Hardware > Boot Order.