A subplot that is merely a second plot is a structural liability: it takes time from the first and pays back nothing. A subplot that works is one where the theme is examined from an angle the protagonist cannot see, usually through a character who is making the opposite mistake or the same one further along.
The clearest test is intersection. If the subplot could be lifted out without changing the main story, it is a parallel piece rather than a subplot. The two lines have to touch — ideally at the midpoint and again near the climax — or the audience is tracking two things for the price of one.
In short forms there is usually no room for one, and inserting a subplot into a two-minute film is the most common way to make it feel rushed. What short work uses instead is a single character who embodies the counter-position.
What is a subplot for?
To examine the story’s theme from an angle the protagonist cannot occupy, usually through a character making the opposite mistake. A subplot that is just a second plot costs time and returns nothing.
How do you know if a subplot is working?
Try removing it. If the main story is unchanged, it is a parallel piece rather than a subplot. Working subplots intersect the main line at least twice and alter what the ending means.