External Subtitles would be problematic for stacked movie files, as the first part of the file name for an External Subtitle file must match the file name of the movie file.
So you would need two External Subtitle files, one for each part of the movie. You would select the correct subtitle file for the first part of the movie, and when the first part finished you would need to select the correct subtitle file for the second part of the movie. But you should only have to do that once, the first time you watched the movie, or perhaps in a quick run through before you sat down to watch it.
Also, finding External Subtitle files for split movies could be quite difficult. I think you would have to find one for the whole file, then split it, then adjust the timing for the second file. MC has a function to adjust Subtitle timing, but I don't think it would have the range to adjust for half a movie. There are tools to do that gross level of adjustment available though. Search the internet if you need one.
Note that Subtitles can be selected, and timing changed, using the OSD menu as well as the right click menu. Just arrow up on your remote or keyboard to find the functions.
Also, search the Wiki and Forum. There is plenty of information on External Subtitles.
Bottom line: If you have a split movie and need to use External Subtitles, it would probably be easier to join the split movie first.