How to merge MTS files (movies) which have not been imported with PlayMemories Home?
The PlayMemories Home software can combine several MTS files if it detects that they are a part of one recording. For this to happen, the footage has to be imported directly from the camera using PlayMemories Home.
If the files have not been imported with PlayMemories Home, the best that you could do is register the folder containing the MTS files with PlayMemories Home and combine them using the [Combine videos] feature.
You can always preview the videos to be combined so that you know their correct sequence. Further information about operation can be found in the PlayMemories Guide: Combining Videos with PlayMemories Home