How to split subtitle for 2 CDs

I have downloaded some subtitles from the internet after I read some posts in this forum. The subtilte displayed fine in both BSplayer and zoom player pro, however, I don't know how to split it into 2 CDs. It has a .sub extension but it doesn't contain a idx file with it. Though it displayed fine with bsplayer, but vobsub cutter doesn't allow me to split the file without a idx file. And I have no idea which software they use to create that subtitle file, since the file size is really small (58k) compare to the usual subtitle file I created with Vobsub. Does anyone has any idea how to do that?