You have to split the ownership if you want 12 disks used for each node.
You can have an odd number of disks in a raid group, that's no problem. Whether you have 1 or 2 spares is a personal choice. One spare would give you more usable capacity. If you know you could get a replacement disk quickly - then 1 spare should be fine. If it might take awhile to get a replacement on site, you may want to have more than 1.
I would split the shelf - odd #'s to one controller and even #'s to the other controller. Since the SSDs are so expensive, we chose to go with #disks+2DP with 1 spare. The rebuild times are extremely fast on the SSD raidgroup. You would essentially have to lose 3 disks within 4 hours if you only have 4 hour support. Never had a SSD fail in my experience.