To expand on kindnugs post.
I doubt your ever gonna find A direct copy of either but who's to say what og18 and kosher kush exactly is. Holy grail is a great mixture of the 2. I got an absolute winner pheno out of 1 pack (extremely lucky) Its a good blend of the 2. og18 and kosher kush arnt very unrelated they have a lot of the same traits to begin with so the variation in these beens will probably be minimal. out of 5 i got 5 that were very close with 1 just out yielding BY A TON and it seemed to hold its smell/flavor a lot more through the drying/curing process. If I'm not really thinking when i puff I've had the thought.... "is this OG18?" or "DAMN this taste just like that kosher i had!" Its a really AMAZING strain if your a fan of og18 or kosher.
Think of it like this. If SHAQ and Alyssa Dehaan from MSU were to have kids. Some would be more white some would be more black but most are gonna be a mix in between. Some females are gonna look just like SHAQ and some Males are gonna look just like Alyssa. MOST are gonna be tall and probably won't be able to shoot free throws. Your not gonna find exact copies of Alyssa or SHAQ in the mix but you might find some very close copies if they are raised in a similar manner.
Breeding is like painting with water balloons. You gotta get REALLY lucky to paint the mona lisa but its possible. If you go into the painting without trying to "Find" the mona lisa who knows what beauty the water balloons will come up with!
and if you really wanna paint that mona lisa you better get a shitload of canvas/paint/balloons (aka seeds) And start practicing!