To me, those two faces dont look very elven, and I always thought theyre supposed to be represantations of the paths that the Bhaalchild could follow - i.e. the "dark" and "light" side (embracing Bhaal's essence or rejecting it).
But then, its probably just me.
I was also surprised that the cover from Icewind Dale 2 is supposed (according to postings on the Iwd2 board) to picture the "twins", while Ive just interpreted the guy as a generic tiefling and the gal as an asimaar (or elf, or half-elf...basically just showing off the novel PC races) since they dont share that many similarities with the ingame avatars of the twins (red skin or wings frex).
Of course it could just be that the cover was designed before the developers had made a final draft about the twins' appearance.