From what I understand, most people don't really study for Step 2 CS, and if so then not very much (especially if you have been doing OSCEs throughout medical school). So I don't think it would be particularly difficult to take them within a week of one another. If you do this, I would do CK first so you don't waste a day or two on CS stuff when you could be preparing for the harder (and much more important) CK exam.
My question for you, though, is why would you do this? Is there a particular reason to take them within a week? I can see taking CK at a certain time to include it in your residency application, but CS is simply pass/fail and doesn't really matter to anyone.
I took them a week apart, no problem. It's great to get it all out of the way. I didn't study for CS more than 2 days (and that was just skimming through the First Aid and going through cases).