I was able to do some research and unfortunately we currently no longer have the variant with 2 HDD slots in our range. Unfortunately, it is not possible to predict if and when it might come back, as the corresponding purchase offers are currently missing.
Yes, the appropriate SATA hard drives must be purchased separately. The NAS is supplied as an empty housing without hard drives, so you will have to install the desired 3.5-inch SATA hard drives yourself.