With the firmware version xfvdw-sl-15-3-2big1-30may2014 it is possible to add the second disk.
You have to place the drive containing the firmware into the left hand slot of the 2BIG1 (seen from behind).
Set the rotary switch to the "big" position.
Look at the "First time install instruction 5big2 2big2 isp2, nwsp2max cloudbox 2big1-30may14.zip" description
in http://plugout.net/viewtopic.php?f=7&t=1867 for disk formatting.
PS: RAID configurations are not yet supported in the version 15-3.