Halloween dosn't have 2 W's so when splitting the word to 2 lines wouldn't hallow - een make more sense?
The "een" part is an archaic contraction of "evening", or more specifically "eve", as in the evening before All Hallow's Day (November 1). The traditional spelling is Hallowe'en as a result. So yeah, separating the "een" (or "e'en") makes the most sense if breaking across a line.