In Winesburg, Ohio, who are two characters who merit comparison and contrast? (Contrasting George W and Elmer wasn't very fruitful.)

I agree: any parallels between George and Elmer just aren't that interesting. Since there's a huge cast of characters in this book, let's try some other pairings.

How about Louise and Emily? They're both unhappy in their marriages, but for different reasons, and this pairing will help you bring out the theme of dissatisfaction and disillusionment that's so prevalent in the book.

Then there's Wing and Alice.They both seem unable to express their sexuality...

I agree: any parallels between George and Elmer just aren't that interesting. Since there's a huge cast of characters in this book, let's try some other pairings.

How about Louise and Emily? They're both unhappy in their marriages, but for different reasons, and this pairing will help you bring out the theme of dissatisfaction and disillusionment that's so prevalent in the book.

Then there's Wing and Alice. They both seem unable to express their sexuality in a way that society will accept. 

Both Curtis and Jesse seem deeply devoted to religion, yet they stray morally in different ways.

Enoch and Seth are both hopelessly misunderstood, and therefore withdrawn from society, but for different reasons. Like with the Louise/Emily pairing, this particular pair would help you build in some discussion of that theme of dissatisfaction with the world.

I found the pairings listed above by consulting a list of characters and looking for things they had in common, which is what you can do if you'd like to keep searching for a good pair. But you could also simply pick your favorite story (does any one in particular seem to pluck at your heartstrings?) and then select a pairing for that main character from there.