On showing two or more subtitle tracks:

• KMPlayer (Windows): Allows you to load multiple tracks loaded from external subtitle files, but does not seem to allow showing more than one of the tracks included in a DVD.
• On Windows, one can let the ffdshow filter display one track, and have the Media Player display another. This is essentially what this article suggests. This does’t work with DVDs either, they never seem to play via ffdshow.
• PowerDVD allows choosing a secondary subtitle track, though apparently this is only supported when playing DVDs, not for Blurays.
• 2srt2ass – webservice that makes it easy to merge two .srt files.
• VLC Ticket: Ticket in VLC tracker to support this feature.