DL380p Gen8 - Requires Additional Controller for optional +8 SFF Cage?

[PDV HP]

Per the quick specs for HP Proliant DL380p Gen8 

"NOTE: The optional +8SFF drive bay requires an additional internal storage controller be selected. HP recommends the HP Smart Array P420/1GB FBWC 6Gb 2-ports Int SAS Controller (P/N 631670-B21)."

Per this Smart Array product page the internal HP Smart Array P420i/P420  max drives is "Up to 27 internal".

Why is a second P420 controller required for 16 SSF (8 SFF and optional 8 SFF drive cage) drives?

Thanks,

Dan

[PDV HP]

Answered my own question.  Further in the quick specs I found this:

NOTE: In Gen8, the 8+8SFF solution is designed as a performance
configuration delivering maximum IOPS across the two drive cages. Customers
seeking to have all drives in a single volume should consider the DL380p Gen8
25 SFF Configure-to-order Server (P/N 665554-B21) as a base.