You will need two of the same item or hero in order to Star Up (upgrade) them. However, they must be of uncommon or better rarity and both must meet a minimum level requirement. The number of stars you can get depends on rarity. An Uncommon can result in 2 Stars, Rare to 3 Stars, Epic to 4 Stars, and Legendary to 5 Stars. Starring Up your Heroes will result in a stat boosts and a new ability. Continually upgrade the buildings in your town to stock up on resources and keep production active.