What language techniques do the quotes "All men are enemies. All animals are comrades" and "Four legs good, two legs bad" show?

While the above clearly explains, I really appreciated this part because that quote summed up one, and yet two, pivotal points of the story.
That quote is what gives the animals something to separate themselves from the men. It gives them a boundary and then unites the animals by being similar on one side of the statement with no way of being 'in between', no way of allowing one of two legs to be apart of the four-legs "club", or vice versa.
Like TeacherSage said, these quotes use antithesis- using complete opposites to show the characteristics of each side.
The first pivotal part of the story was that distinction, the "us vs them" mentality.
The next key part is where, despite the very firm line, "us" and "them" becoming mixed up when the pigs began breaking the other rules that were designated to separate the two sides. This continues until the final statement, "... from pig to man, and from man to pig ... but already it was impossible to say which was which.”
Finally, with that statement, the story comes full circle with the distinction clear again.

These quotes use the language technique of antithesis. Antithesis is the juxtaposition or putting together of opposites to make a point through contrast. In this instance, animals are contrasted to humans. We learn first that all humans are enemies of animals. Second, we learn that all animals are comrades, or friends. A comrade is a person you are in community with and loyal to.
In the second quote, the four legs of animals (bird wings count as legs so that birds can be included) are contrasted to the two legs of humans. Again, the contrast is stark. There is no room for compromise. Four legs are are a good thing to have, two legs bad. The quote means animals must stick together: they have nothing in common with humans.
These quotes are also examples of antithesis because they use parallel grammatical structure. The parts of each quote are set up identically, following the pattern, first, of "All [blank] are [blank]," and second, of "[blank] legs [blank]." This structure highlights the contrast between animals and humans.
The quotes are also aphorisms, which are statements of wisdom put in a short, memorable way. It is easy for the animals to remember these words, both because the statements are short and simple and because the grammatical structure is parallel. In fact, the sheep will learn to repeat the second quote—"four legs good, two legs bad"—over and over to drown out dissent.