To get double the performance and double the capacity you can opt for RAID 0 (Striping).
After installing the 2nd HDD into the J2i module and cabling it up follow these simple instructions:
Step 1. Open your disk utility and run the RAID Assistant
From here select either RAID 0, RAID 1 or JBOD. We will select RAID 0 for performance and capacity.
Step 2. Select the two J2i drives
Step 3. Label the Volume and format Mac OS Extended (Journaled). Recomend to use default settings.
Click Next followed by Done!.
Lets review your configuration. The volume is made of two drives in RAID 0 (Striping) for double the capacity and performance.
Lastly. Lets benchmark the volume using Black Magic Speed Test, we should see double the throughput of a single 8TB HDD. Looks good!