I've had some good fun combining ketamine and cocaine, though the cocaine was snorted and the ketamine IMed. I suspect some synergy between these two.
While it isn't the safest combo, neither is it the most dangerous. Ketamine generally doesn't have appreciable effects on the cardiovascular system so the traditional theory behind uppers mixed with downers being bad doesn't really apply here. Especially if you stick within a low to moderate dose range, as you always should when combining drugs.
As for smoking ketamine...that just seems foolish and wasteful to me. Ketamine is really better IMed or if you avoid needles, then snorted or taken sublingually. But you could certainly snort a coke/ketamine combo or do as I did and IM the k/snort the coke.