Piebald is a condition that gets called Koi Or Marble. Then there are other Marbles that get that trait from a different Gene or genetic condition. Marble is just a broad term that's been thrown at anything with pattern variations.
Now i haven't worked with marbles yet, but in snakes, humans, cattle and other animals, piebald is a recessive trait.
But then there's the other conditions that result in Marbles. "Jumping Gene", "Red -loss gene", those may be Dominant, that's not important right now. Even with a dominant gene in only 1 parent your yield of marbles would only be around 50%
Breeding 2 marbles is your best bet to produce marble offspring, however the problem is that you have to be sure that those fish both are marble as a result of the same condition. Otherwise you may be crossing two separate recessive traits, and have a whole spawn that doesn't demonstrate either of the traits you're after.