Possible Clues:

• Indiana and Purdue, e.g.
• Harvard and Yale, e.g.
• Two sides of a certain triangle
• Coke and Pepsi, e.g.
• Harvard and Yale, e.g
• Airport area overlooks a royal 1 across and 10?
• Competitors, challengers
• Target and Walmart, e.g
• Two teams playing the same sport in the same league in the same town, most likely
• Compares closely to