Generally, it wasn't necessary for ancient armies to agree on where to fight, because it would be all but impossible to evade each other. The location of the impending fight would be gradually narrowed down by factors such as the time of year, the armies' logistical needs, march routes and strategic targets, until finally the site of battle would be obvious to both sides. This is not the same as agreeing on the time and place in advance, which only happened once in Greek history. I wrote more about how ancient Greek armies found each other (and about that single exceptional case) in this older thread.

The scene in Troy is based on the early books of the Iliad, in which the Greeks and Trojans agree to settle the war in a single duel between Menelaos and Paris (as the movie also shows later). According to the poem, this plan fails because the goddess Aphrodite saves Paris just as Menelaos is about to kill him. Zeus then persuades the Trojan Pandaros to shoot an arrow at Menelaos, breaking the truce and starting all-out battle. This is part of a pattern of Archaic Greek stories in which wars are meant to be settled by duels, only for one side or the other to cry foul, after which full-scale battle becomes necessary. I wrote more about these attempts to restict the violence of war, and their failure, in this older thread.