If RAID6 is used, then you're good.Do you think 2 drives will die in the same day?
I generally use 1 spare. And sometimes share that between more than 1 array.
I have never experienced a disk failure during a rebuild in 15 years of running raid 5 and 6. Although I have had at 20+ disks go bad in arrays. I catch most of them now before they die.What happens if you have a disk failure/rebuild one may fail?