Original post by: Dan ,
Sorry for not getting to you, I needed to do some checking. El Capitan Disk Utility is not as good (complete) as the older Mavericks & Yosemite versions. It appears Apple still won't let you create a Fusion drive with it with 3rd party SSD's. But don't despair! You still can do it you just need to do it using the command line interface. Follow this: [http://www.macworld.com/article/2014011/storage-drives/how-to-make-your-own-fusion-drive.html|How to make your own Fusion Drive]. Be aware you will need to do this from an external bootable drive as you can't do it using the OS on the current drive.