"We will see who is the real king" mentioned Kaiba when the final battle begins (between Yugi and the Pharao). And it's a good question. Yugi is known like the King of the game but isn't it Pharao who duels most of the time and wins?
there are 3 answers:
3) both, together they are indestructible
i think is 3) each of them is a strong duelist, seen like a challenge by much opponents but when they join together, you create an indestructible duelist.