* Russian Blue is a breed of cat with a specific coat color and pattern. The breed standard dictates a blue-gray coat with silver tipping, and a specific body type.
* Tabby is a coat pattern, not a breed. Tabby patterns can appear in many different cat breeds.
Therefore, a cat can be either a Russian Blue (with its specific coat color and breed characteristics) or a tabby (with a tabby coat pattern, regardless of breed). A cat cannot be "half" of each, as these are distinct categories.
However, it's possible for a cat to have Russian Blue ancestry and also have a tabby coat pattern. This might happen if a Russian Blue cat was bred with a cat of another breed that carried the tabby gene.
In short: You can have a Russian Blue with a tabby pattern, but you can't have a cat that is "half" of each.