Split subtitles to line up ends of files?
Don't transcode your video, there shouldn't be any need to do that.  You might be able to concat the two ogm videos into a single ogm, or mkv, without transcoding, just remuxing.  (e.g. with ffmpeg's concat filter, and -codec copy)

http://www.nikse.dk/SubtitleEdit has a split feature.  You'll have to edit the 2nd file after splitting, since the split dialog box doesn't have anywhere to add in a gap.  But it's easy to apply the same time-shift to all subs in a file, with it.  I haven't actually used that editor much, I think last time I had to mess around with subs I used gaupol or ksubtile, forget which, (on GNU/Linux) to stretch the timecodes in a .srt to correct for a 25fps / 23.976fps speed difference.  But anyway, I think Subtitle Edit is good, and it's cross-platform.  (Runs on GNU/Linux under WINE, with mono.)