Iām not sure how that solution would help you if you have background music. The music would have to be re-cut or re-timed to match the different video lengths. At that point, you would have two complete and independent projects living in the same .MLT file, which would probably create more problems than it solves.
If I had this assignment, I would create the short video first in its own .MLT file. Once finished, I would copy the .MLT file to a second file, then edit that second file to add the additional clips. Lastly, I would add custom music to both files so the music matches the length. Then export from each file independently.
You could also make the long video first, then copy it and cut it down to make the shorter video. It depends on the nature of your video.
There are a hundred ways to approach this, but one of those two is the route I would go.
If you really wanted to keep everything in the same .MLT file, you could put your short video on V1 and your long video on V2 (or something similar). At the time of export, turn off visibility of V1 or V2 (using the eyeball icon on the track head) to export just the remaining visible track(s). You can selectively hide your short or long video this way.