diskutil coreStorage in Lion fails to create a concatenated LVG, error -69886
Operating system

Can you reproduce the problem with Mac OS X 10.7.5? 

If -69886 arises from a failure to unmount

Expanding upon a hint in an answer to a linked question, try this before the Core Storage operation … for the device that apparently can't be added: 

diskutil unmount /dev/disk2s2

– then if the unmount fails, to discover what's open: 

sudo lsof /dev/disk2s2

Reference

Currently limited by Apple to Mac OS X 10.7.4: diskutil(8) OS X Manual Page