QNAP storage ES1640dc v2 question

Hi Exeprts

The ES NAS will use drive bays 1 to 4 to store the system configuration,
QNAP recommends drive bays 5 to 16 for data storage. This setup prevents data storage operations interfering with the basic operation of the NAS system.
It also means that the disks used for data storage can be easily migrated to another NAS.
When using a mixture of SSDs and HDDs, installing the SSDs into drive bays 1 to 4 is recommended.
 As the QNAP documentation said the first 4 disks will be for OS configurations
But in my case, I have only tow SSD disks which I want to use them to make Configuring Volumes for SSD Cache
So now I am confused in which slots I have to put the SSD disks in order later to create an SSD volume and Configuring this  Volume for SSD Cache

here all links which include info about my question but i don't find   the answer myself

https://www.qnap.com/en-us/how-to/tutorial/article/qnap-storage-performance-best-practice

https://www.qnap.com/en-uK/tutorial/con_show.php?op=showone&cid=280

http://docs.qnap.com/esnas/1.1.x/en/index.html?introducing_qes.htm

Kindly advice
Hussam