From page 40 of the Warcry Core Rulebook:
Once a fighter has used an ability, discard the ability dice used for that ability. You can use a [triple] for an ability that requires a [double], or a [quad] for an ability that requires a [triple] or [double], but if you do so all those ability dice are discarded.
That leads me to believe a [quad] can not be split into two [double]s as it's not presented as an option.
As for if you roll five or even six of the same score on the dice, this is covered on page 37 of the Core Rulebook (emphasis added):
Finally, if 4 or more of your ability dice have the same score, it is referred to as a [quad].