Adding 2 new disks to existing RAID5 array (with 1 spare)

[KoosvH]

I have a Prolaint ML350 G6 server; diskspace is running low. The Smart array controller is P410i, I just installed latest firmware on the controller.

There is 1 array with 5 disks, each 300 GByte and 1 spare, so I have 2 diskslots free. I placed two new disks, same type and capacity, in the empty slots. In ACU they are reported as unassigned. Oke.

Now I have to expand the exitsing array, When expanding is finished I have to extend the logical disk and at least I have to use diskpart in the OS to add the free space to the non-system datadisk (OS = Windows 2008 64 bit).

I have set the rebuild prio to fast.

When expanding in the ACU I can choose 1 drive at a time, or both new drives.

My question: Is it preferable to add one drive at a time, or can I add the two new drives at the same time to the array and let the array expand. Is this save (However: in fact, expanding always is unsave because during the expanding process there is no fault tolerance).

Do I have profit of the spare?

The array is 1.1 TB, I have Cache with battery, so the rebuild is ca. 30 sec / GByte > ca. 10 hours), correct?