Weaker Twin Saves the Day

A pair of Always Identical Twins who are : One's stronger in some way (more intelligent, smarter, quicker, athlete); the other one weaker (less motivated, the disappointment, inferiority complex).

The plot starts with the stronger twin dead or dying. The weaker twin mourns his brother's passing, but suddenly finds that he is thrust into the limelight and forced to Take Up My Sword and assume the mantle of his brother since he's a Suspiciously Similar Substitute. Often, this is due to his genetic make-up being exactly the same as his brother's, and hence, despite the apparent lack of suitability (his lack of confidence, weakness of character, or some other perceived weakness), he is the only possible candidate due to his genetic likeness.

Often the character suffers as the Angsty Surviving Twin but has no choice in the matter as he is the Backup Twin due to his DNA sequencing being exactly like his dead brother's and is essential to the plot moving forward.

Also, this is usually a perspective flip of Backup Twin shown from the perspective of the people who knew the original and recived the back up. This is where the backup is the main character.

Examples of Weaker Twin Saves the Day include:

Film

In Eagle Eye, Shia Lebouf plays a set of identical twins, but the main character is a 'failure' whose twin brother works on a top secret AI project called ARIIA. The brother was killed in an accident in the film. The main character is mysteriously 'activated' by the AI as his identical DNA is crucial to unlocking the AI from a state of inability to act.
In Avatar, Jake is paralysed from the waist down; he survives his twin brother, who works on a scientific mission to Pandora and has a Na'vi avatar which only a genetic equivalent can use. Jake's asked to take up his brother's avatar (otherwise it's a waste of an investment), but instead, Jake sides with the Na'vi.
In the Chris Rock vehicle Bad Company, a CIA agent dies in the middle of an important undercover operation, and the CIA call on his separated-at-birth twin brother, an immature hustler, to keep the cover identity alive.

Literature

In the Robert Heinlein book Time for the Stars, the Long Range Foundation is sending Generation Ships to the stars, and to overcome the difficulties of time-dilated, luminal communication, they take advantage of the occasional ESP of identical twins. The hero lives in the shadow of his more out-going, adventurous brother, but ends up going to the stars when his brother breaks his leg skiing.