Recently I got an .MKV HDTV file with MPEG-4 AVC (h264) video and two AC3 audio tracks. The bad thing was that the .MKV file was 10MB larger than a single layer DVD disc, i.e. it was 4,38 GB instead of 4,37GB I could fit into the DVD disc.

The problem was that many applications still didn't support files larger than 4,37. Also MPEG-4 AVC (h264) is not always supported even with a single audio track.

I didn't want to re-compress the video. Splitting MKV was the only option. Luckily the video had a dozen of useless seconds at the end.

• I tried freeware avidemux (Windows) first. It was slow, didn't seem to support 2 audio tracks, besides I got several dialog windows asking me about indexing and other stuff that I didn't really want to see.
  
• Then I tried freeware mkvtoolnix (Windows, Unix). It did the job perfectly!
  
  mkvmerge GUI from mkvtoolnix has some disadvantages:
  • you can split after a given amount of time, but you can't split after several given amounts of time;
  • you can't preview the video in the mkvmerge, so you need to launch a player and remember the time when you need to split.
  
  But it is fast. It support MPEG-4 AVC even with two audio tracks, so I recommend it!