I assume we're talking about two textbased subs like .srt files.
I also assume you're talking about two joined/merged videofiles and that the two .srt-files you have were fully synced to the two original videos.
Now, I've never joined/merged two subs using BS.Player's in-built subtitle editor (I did it using several methods in Vladimir's subsync though), but I think it isn't a big problem and could easily be done in a similar way.
(It wouldn't be a bad idea to keep copies of the sub-files in a safe place.)
(1) Note the length in time (
duration) of the original first part of the movie (I suppose you still have it)
(2) Load (load new) the sub-file for the (original) second part of the movie in BS.Player's subtitle editor
(3) Insert the
duration as the "Delay" under "Adjust delay" (bottom left)
(4) Click on "Apply" and then Save the adjusted subtitle
(5) Try if the adjusted subtitle is in sync with the second part of the (merged/joined) video file.
(6) If not redo the above steps (changing the duration in accordance with your findings)
Only when the adjusted subtitle is fully in sync with the second part of the movie (there will be no subs during the first part) you can proceed: (a) Load (load new) the adjusted subtitle file in BS.Player's subtitle editor
(b) Select the very first line by a single left-mouse-click on it
(c) Now click "Subtitle" => "Load subtitle" => "Insert before selected line"
(d) Browse to the sub-file of the (original) first part of the movie and doubleclick on it
(e) Save the now joined/merged subtitle
That should do the trick.
