Because then there would be two different places that could possibly be the epicenter. If there was a third, then they could find the exact epicenter. Think of it this way... If you took a piece of paper and drew three circles that all overlapped each other, the point where ALL three of the circles meet is the epicenter. Now draw 2 circles that overlap. There are two different places where they meet, you wouldn't be able to know which is the epicenter.
The seismic waves originate from focus-a location on the earth that moves so as to send the waves. Above the focus is an epicenter therefore if the waves are coming from two places, I think it would be difficult to get the epicenter unless by the help of a seismogram.