Joining/merging m2ts/mp4 and other formats

What's the best app that can join m2ts/mp4 format files into one, without changing video/audio quality. And if there is any that can make a "history file" so if you in the future want the files split again, that it makes them look just as they where from the beginning.

Now I know some files will be recorded in different video setting/quality and also maybe audio. How will this effect the one file?