Hey,
It's OK to do, but eventually the finer grained sand will end up under the cc. The look you end up with will depend on how much of each you have.
I've got about 80% sand and 20% cc just so my fish have something to play with.
I think it just looks more natural. I also have pieces of lr rubble scattered around. HTH
BTW, there are purists who will disagree with me and think you MUST have a pure DSB to have a successful tank. IMHO that's bunk. Just my .02.
Dan'l