Windows 2012 R2 Storage spaces - even number of disks with mirror?

Hi,

I'm just testing Windows 2012 R2 storage spaces on a lab with an MSA70 containing 25 SAS disks.

So I created a storage pool with the 25 disks, then tried to create a mirrored virtual disks and it failed telling me that I did not have the appropriate number of disks.
I removed one of the physical disk from the storage pool (now down to 24), and was then able to create a two-way mirror virtual disk!

I thought that the two-way mirror functionality in Storage Spaces would be writing block randomly on two disks, and not specifically "link" two physical disks together in a mirror.

Is that not the case? why do I need a even number of disks when creating a two-way mirror?

Thank you,
Stephane