Softraid 6: OWC Mercury Pro U.2 Dual with 4x ADATA SX8200 2TB very slow performance, unstable
Each bay in the U.2 Dual is two PCIe lanes
Each bay of the U.2 dual is x2, so a total of x4 (2+2). The point being is to use slot A and B in RAID0.
Since the total bandwidth of Thunderbolt is x4, two x2 drives in RAID0 will achieve a total of an x4 uplink. I'm presuming you're using two Shuttles (4 blade slots per Shuttle) for a total possible configuration of 8 blades? If so, a x4 to each bay would be overkill, since each blade would only be able to provide a subset of that before saturating the Thunderbolt bus.
Described differently, put one Phison blade in Slot A of the U.2 dual. Put the other blade in Slot B. RAID0 and you'll achieve full x4 bandwidth. The dual+shuttles combo allows you to use up to 8 blades, so 8x the capacity at a total of a x4 uplink. Does that make sense?