I wouldn't want to be the person trying to figure out which hatchlings were ghost type 1, type 2 or both type one and 2. Unless the incompatible lines compliment each other and there is a significant change in the amount of melanin expression I would think breeding out would be the only way to declare a snake to have 2 copies of both genes.
Maybe someone has tried it and the results were not stellar or perhaps due to it being a problematic double recessive combo to identify none have yet tried it beyond testing for compatibility with other lines.