So if Isochron Scepter's enter the battlefield trigger is copied, and it imprints two spells, when you activate its ability does it cast both or one of them?
You copy both and may cast both, one, or neither.
"In some cases involving linked abilities, an ability requires information about "the exiled card." When this happens, the ability gets multiple answers. If these answers are being used to determine the value of a variable, the sum is used. For example, if Elite Arcanist's enters-the-battlefield ability is copied, two cards are exiled. The value of X in the activation cost of Elite Arcanist's other ability is the sum of the two cards' converted mana costs. As the ability resolves, you create copies of both cards and can cast none, one, or both of the copies in any order."