Every sumo bout ends with a winner declared.
Even if the gyōji (referee) thinks both men crashed to the ground at the same instant, he is obliged to point his gunbai (war fan) toward either the west or east to signal that the wrestler from that side has won.
The five ringside judges, who are the ultimate authority, can overrule that decision if they believe the gyoji got it wrong. They may also call for a rematch if a clear winner is not obvious.