The dragon has two crest genes, rather than one. You may choose any crest that the dragon does not already have (other than ‘no crest’). Here are several examples:
It must follow elemental color rules. You may allow display traits to affect it.
Modifier & Physical Trait Interactions
The crests still follow normal crest rules. However, you may choose for each crest to have different colors / treat them as separate crests.
Charged Double Crest
A Charged Double Crest becomes a triple crest.