Cards have the same artist if there's an artist in common among them. For example, you could exile four cards illustrated by Zoltan Boros and one card illustrated by Zoltan Boros and Gabor Szikszai. However, you could not exile two cards illustrated by Zoltan Boros, two by Gabor Szikszai, and one by both of them, as those five cards don't have a single artist in common across all of them.