sand and cc do not mix well, first as mentioned ti seperates and the sand will more than likely sift thru to the bottom anyhow,
2d mixing them, kinda defeats the purpose of having either
the benefit to cc is looks(some like it) and filtration and cleaning is easier(adding sand would complicate it), andother is to only need a lesser amount
the benefits to sand is looks(again some like it), cleaning is not existant if done propelry(but you cannot use a ugf and should not vaccuum), it also works as a bio filter where adding cc greatly lessens its effectiveness(unless you wnat to us 12 inches or more, the ratio is aobut 1:4 i believe)
so i would only use one