If you are asking if a WD Sentinel has two hard drives and one were to fail, will the other hard drive keep working? The answer is yes. See page 31 of the of the User Guide you linked to above.
There is 2 units sentinel with 8TB, and concern about if 1 unit sentinel 8TB failure, will the system automatic fail-over to my second unit of sentinel 8TB /
Do this sentinel have this kind of features?
As i understand sentinel come with 2 power port, 2 network port , either each port (Power or Network) failure, it will automatic fail-over to the second port to avoid down time.
Hope above statement would bing you more clearly on my concern. Waiting for your prompt reply. Thank you.