Because you can look at it two ways and see in the same direction

Maybe because it can be a mirror in either direction depending on which side is lit/dark?

Corridor Digital mentioned them recently in one of their videos and had a 5-minute conversion on what the correct term was before finding that Wikipedia page.

I guess you can look at it from two sides (in two ways?)

It’s a terrible name. Two-way because you get the normal mirror reflection and a see-through. Should’ve just called it a see-through mirror, it would immediately make sense.