Thunderbolt Duo: multiple partitions on RAID set

Apologies if this is the wrong category. I can't figure out where to post this.

I have a Western Digital Thunderbolt Duo. Right now, there is one big 4TB partition and I would like to slice off 250GB to create a clone of my MacBook Air SSD in case something goes wrong. I've looked at Disk Utility and what I see there is the following :

What I think this means is:

1. Each of the physical drives is known to OS X, and contains something called a 'RAID slice'
2. The 2 RAID slices combined create a logical volume called 'seven'
3. The logical volume contains a partition called 'Images'

So, what I really want is for the logical volume to contain 2 partitions. The existing 'Images' partition, but reduced in size. And a 250GB partition for the clone of my SSD drive.

When I go to the logical volume and hit the 'RAID' tab, it shows that it's built from the 2 slices and even allows me to create a zero byte RAID set. But there is nothing I can do to further create a partition.

When I saw this, I thought that maybe if I created additional 'RAID slices' on the physical drives I could then attach them to the new RAID set. And although the physical drive has a Partition tab, I can't reduce the existing and add an extra partition to it.

So ... I'm lost. I think if this is at all possible, it would include backing up the drive and messing around with the configuration. But before I do that, I figured I'd ask around here to see if anybody knew what to do about this.

(WD Drive Utilities doesn't provide any help either)