A simple 3D subtitles convert, convert SRT to ASS format, work well with MPlayer and VLC.
This tool is made for GNU/Linux but is reportedly working fine with OS X (previously known as Mac OS X).
To convert a subtitle to Side-by-Side
sub3dtool input.srt --3dsbs -o output.ass
Whereas input.srt is the path to the file you need to convert.
output.ass is the name of the file you want as the result.
--3dsbs indicates that you want to convert the subtitle to 3DSBS (Side-by-Side) format.