Don't fall for it, it's a classic blunder! Promoting to king makes it twice as easy to get checkmated! Article 1, section 2 of the FIDE Laws of Chess, states:

The objective of each player is to place the opponent’s king ‘under attack’ in such a way that the opponent has no legal move. The player who achieves this goal is said to have ‘checkmated’ the opponent’s king and to have won the game.

Note that it specifies "king" and not "kings" - the opponent only has to checkmate one of them to win!