Usually, the reason why encoders split the large file is to fit into recordable DVD-R back-up. I dont like split files personally but at least it forces me to take a short break for stretching or toilet.
Actually, it's the uploaders who usually split the files. I find some splitted files uploaded in hdb.ro. Most encoders I know don't split their files into DVD sizes. The scene groups split their files into several rar files while non-scene groups encode in only one file. Splitted files are really not cool.
how do you attach subtitles to 2 mkv files?
2 rin sir ung subtitle file, pano iattach to sa mkv file? 
If my understanding is correct, you have 2 mkv and 2 subtitle files, and you want to join everything together for flawless playback. If this is what you want, you can start by joining the subtitle files using software such as Subtitle Workshop
http://www.urusoft.net/download.php?lang=1&id=sw.
After that, you can use mkvtoolnix (mkvmerge gui)
http://www.bunkus.org/videotools/mkvtoolnix/downloads.html to merge them all together. Press 'add' and choose the first mkv file then click 'append' and choose the second mkv file. Afterward, click 'add' and choose the joined subtitle file (e.g. *.srt). Press "start muxing" and wait until a new mkv file is created.
Clondalkin's suggestion will also work if you don't want to integrate the subtitles to the movie file itself. Join the 2 subtitles together using Subtitle Workshop. Then join 2 mkvs together using mkvmerge gui. Make sure the subtitle files have the same filename as the joined mkv. Viola, you can watch the movie with subtitles. But I prefer to mux them all together in one file. You can set no subtitle as default in mkvmerge by the way.
Good luck!
