How would your reaction (to events in the plot) be different if you didn't know what both characters were thinking?

This story certainly employs an omniscient narrator. So, the reader does get a clear idea about both Georg's and Ulrich's feelings towards one another. 

The narrator gives the details of the longstanding feud. And the narrator adds the way both men think about one another: "as boys they had thirsted for one another’s blood, as men each prayed that misfortune might fall on the other." Then, the narrator provides their thoughts as they approach one...

This story certainly employs an omniscient narrator. So, the reader does get a clear idea about both Georg's and Ulrich's feelings towards one another. 

The narrator gives the details of the longstanding feud. And the narrator adds the way both men think about one another: "as boys they had thirsted for one another’s blood, as men each prayed that misfortune might fall on the other." Then, the narrator provides their thoughts as they approach one another: 

If only on this wild night, in this dark, lone spot, he might come across Georg Znaeym, man to man, with none to witness—that was the wish that was uppermost in his thoughts. 

The two enemies stood glaring at one another for a long silent moment. Each had a rifle in his hand, each had hate in his heart and murder uppermost in his mind.

This intense build up of hate, animosity, and murder makes the inevitable truce that much more incredible. Without the narrator's insight into the incredible hate these men have for one another, the shock of the peaceful truce would simply be much less dramatic and less interesting. Also, without these insights into the men's thoughts, the reader would not assume that they wanted to kill one another. Upon their meeting in the woods, one might expect nothing more than a verbal disagreement about land ownership. The characters' thoughts are quite important in establishing their motives and in setting up the dramatic reversal when they become friends. 

After they are pinned beneath the tree, the dialogue does suggest the kind of hate alluded to in their previously revealed thoughts. But without those previous thoughts, the reader might assume that their mutual anger has emerged from their frustrations of being trapped under the tree. Without the insights of their thoughts, we don't know how much they really hate one another. And without that foreknowledge, the dramatic reconciliation would simply not be that shocking.