I’d say it’s not required – there will be a couple of references to events from C.A.E.C.O. but nothing that really impacts the story for Darkkin Queen, at least as far as I remember.
Having said that, I would still suggest reading them in order – I found I enjoyed C.A.E.C.O. a bit better than Darkkin Queen, maybe because I still find Caeco a more sympathetic character than Tanya.
You’ll definitely want to read it before the new Compendium comes out. I have a hunch that some of the characters introduced in C.A.E.C.O. will be making an appearance in the Compendium tales…