Symbolism, Imagery, Allegory
Just because Austin and Lee can be seen as symbols doesn't mean they shouldn't be viewed as living, breathing characters. These two men have very real desires and conflicts, and their function in the play is not just a symbolic one. However, if you look close enough, they can be seen to symbolize different aspects of the human psyche.
If we were to go all Sigmund Freud on this thing (which, we're totally about to do), we could say that Austin symbolizes the ego and Lee symbolizes the id.
From the get go, Lee moves through life based on the "pleasure principal," which is pure id. He wants to do what he wants to do and his drives are basic ones. He does what it takes in order to survive, regardless of how it might affect others. He steals or bets on dog fights to make money, he drinks and eats what he wants, and based on his relationship with the "female botanist" and the way he can't remember where a woman he wants lives, it's safe to say that his relationships with the opposite sex are based pretty heavily on sex as opposed to long-term relationship type stuff.
This leaves Austin as the somewhat rational ego. He takes others' needs and wants into consideration, and he has a pretty firm grasp on reality (a key to the ego).
"The ego understands that other people have needs and desires and that sometimes being impulsive or selfish can hurt us in the long run. It's the ego's job to meet the needs of the id, while taking into consideration the reality of the situation." (Source).
Breaking down the way these two brothers approach the art of writing is the perfect way to view them as symbols of the id and ego. Austin studiously takes notes and creates outlines. He's concerned with structure and the realistic qualities of a story:
Austin: It's not like real life! It's not enough like real life. Things don't happen like that. (1.4.71-72)
On the other side of the coin, Lee's approach to writing is to throw out ideas as they come to him. He knows the story he wants to tell, but for him it's mainly in the form of grand ideas. He struggles to engage in the actual writing process. He wants his story his way, regardless of what anyone else might think, but he needs someone else to actually get his story down on paper:
LEE: You write me up this screenplay thing just like I tell ya'. I mean you can use all yer usual tricks and stuff […]. But ya' gotta write everything like I say. (2.8.276-279)