Subtitle Workshop, a nice and free subtitles editor and synchronizer, can also help you split easily your subtitles, for instance, if you have a movie in two video files and you did not manage to find subtitles in two parts but only in a single file.

The sub files can be cut in 2 and the time references in the second part are recomputed to begin with the second video file.

First load your subtitles to Subtitle Workshop (Menu > File > Load). Then in Menu > Tools select the Split Subtitle command, which will open a dialogue box that will let you choose a mode to split your subtitles.

Try first the End of Video option, and select the first video file of your movie (click Browse).