Dell storage scv2000 spare disk and space

We have Dell storage scv2000 series. There was a problem with the disks. Two disks was failed and storage was down into emergency mode. We use RAID 6-10.

Before it happened in the system there was one spare disk.

We replaced failed disks. But storage uses both disks as spare.

Why storage don't used one of the new disks for Raid rebuild?

Also i have question about space. The system shows that 87% (14TB of 16TB) of space is occupied, although there is still 64TB of non-allocated space. What will happens when 2TB of 'free space' runs out? Will the storage use non-allocated space or will it enter into the emergency mode?