You can double the maximum current capacities of transistors, diodes, and some IC’s by adding another one in parallel. Similar to how two LEDs in parallel use double the current, or two batteries in parallel can supply double the current.
Driving high-current loads usually generates heat. In general, more current means more heat. Heat reduces the efficiency of the driver and can even destroy it. Some chips like the SN754410NE H-Bridge can also be put in parallel to handle more current and dissipate more heat. Note that stacking one on top of the other is the same as wiring in parallel! (From http://www.acroname.com/robotics/info/articles/drivers/drivers.html#e19)