AniDB

Shakugan no Shana files 213829 and 213828 (h264 and XviD subs by Eclipse) were mistakenly added under Episode 20 instead of Episode 21.

Nevermind. From skimming the files, it looks like the "new footage" is just an intro and opening sequence and some eyecatch and title screen stuff to splice the two episodes.

I don't know if this will have any effect on the way this is resolved, but supposedly the DVD rips have additional footage not in the TV versions. So the 1&2 release is not really the same as watching TV episodes 1 and 2.