If they put it up as two separate subtitles, the text would have flashed by too fast to read unless you hit the pause button.
No matter how you look at it, the two ways have the exact same drawback: requiring pausing from the watcher to see the text.
I could suggest a third method... the first line of text appears above, the second line of text appears below, then the first line fades out after. That way, it doesn't move, and the text stays up long enough to read.