A Branched Spark is an anomaly that occurs once in a million Sparks. Instead of one Spark, two are created. Though they gain individual characteristics, they share a close bond through "vicarious perception", and feel each other's emotions and pain. They are also linked in life and death, if one dies - they both die.
IDW Generation 1 Continuity
Last Stand of the Wreckers
The Autobot Wreckers, Topspin and Twin Twist shared a branched spark. Both frequently felt the other's pain. Topspin's sacrifice to activate Aequitas ultimately killed them both of them, but it ultimately stopped his brothers suffering.
More Than Meets The Eye
When infiltrating Delphi, the Decepticon Combiners, Sonic and Boom claim to be Genericons with a Branches Spark. As such they were eventually locked up together. which turned out very badly for Delphi and it's patients.