Here's what I do.
I open up the first subtitle with notepad and scroll all the way down just to see how long it is. That gives me a reference point in the video file to look for where the second subtitle file should begin. Remember at what point in the film the 1st subtitle ends.
Then name the second .srt subtitle file the same name as the .avi file and put them in the same folder.
Then open that (2nd) subtitle file with SubtitleWorkshop. It should load the video automatically.
Now go to the time in the film where the second subtitle should begin. This is usually around the length of the 1st subtitle. So look for the first words of the second subtitle. When you find the point in the film where the 2nd subtitle begins, write down the time when the words appear in the film.
Let's say it's 45m23s800ms. That's where the 2nd subtitle should begin, but let's say that your 2nd subtitle file begins at something like 1m20s800ms. Therefore you need to add the difference in time to every subtitle line in that file. The difference is 44m03s000ms.
Now hit CTRL+D. This will bring up the delay option.
Input delay every subtitle by 44 minutes and 03 seconds. You may have to do some fine tuning, like adjusting the delay again by +/- 500ms so that the subtitle comes in right as the person speaks.
Then save the .srt file (Save)
Now open the newly saved 2nd subtitle file with notepad. Hit CTRL+A which will select all. Then copy the highlighted text (Right Click + Copy).
Now open the first subtitle file again, scroll to the bottom of the file to just under the the last subtitle line.
Now paste into the bottom of subtitle 1 the text you copied from subtitle 2.
Click SAVE
Now your new subtitle 1 is merged with subtitle 2 with the correct lines, however you need to open the new merged subtitle file again with SubtitleWorkshop. This will allow SubtitleWorkshop to renumber the subtitles correctly. Click Save again.
Now you should have a good merged subtitle file for your video.
SubtitleWorkshop:
http://www.urusoft.net/downloads.php