Shakespeare establishes the difference between the two pairs of lovers or the two couples very clearly. Hero and Claudio, a young fair maiden and a heroic soldier fall in love in the traditional expected way. Claudio is soon seeking Hero's hand in marriage.
Beatrice and Benedick, on the other hand, are older and very different. Beatrice once loved Benedick, but he broke her heart and is now an avowed bachelor. She is looked upon as a spinster. Although they appear to not care for one another, by sparring or fighting all the time. Beatice and Benedick have a deeper relationship connection that the young Claudio and Hero.
I would also say that the lovers are contrasted in terms of realism. Beatrice and Benedict are the more realistic pair, suffering the ups and downs of a relationship. At the party, Beatrice says about Benedict's heart that "he lent it me awhile; and I gave him use for it." So they were in a previous relationship, one where she feels that he abused her by winning her with "false dice." Benedict has left their relationship so bitter that he calls Beatrice a "harpy" and says he would rather go to the "world's end" that spend one minute talking to her. Yep, that was a very ugly breakup. And that realistic post-breakup hatred leaves them bitter and suspicious, which is why at the end when the deceit is revealed, they are both cautious of admitting their love and opening themselves up to being hurt. Beatrice's claim that she loves Benedict "no more than reason" shows that very human fear.
However, after Claudio publickly disgraces and humliates Hero, she never shows any negative emotions toward him. Even after admitting that she "died defiled," she had no problem marrying him. In fact, when she is brought out to marry him again, she tells him, "And when I lived, I was your other wife," reaffirming her faithfulness to him. If Benedict had unjustly accused Beatrice, she would have made him suffer, but Hero is totally submissive and accepting of even the most greivous insults.
I understand your question, but I'm not sure if you mean in Act 1, scene ?, or Act 2, scene ?. It's important when you ask questions like this to be specific about both the act and scene number so that the eNotes editors can be of more assistance.
I would say, though, that the differences between the pairs - Hero/Claudio and Beatrice/Benedick - are established right from the beginning of the play. Hero hardly has anything to say, and it's up to a director if they're going to have her noticing Claudio much from the beginning or not. We kind of assume they know each other and are twitterpated by one another because of Claudio's words to Benedick in that first act - "Can the world buy such a jewel?" when he is speaking about Hero. Then in Act 2 we find that he is so crazy about her that he is jealous and angry when he thinks Don Pedro is actually wooing Hero for himself, rather than for Claudio like they agreed.
While all this is going on, Beatrice and Benedick have already verbally sparred with one another, both of them claiming that they have absolutely no interest in ever falling in love or having someone fall in love with them - (Beatrice) "I'd rather hear my dog bark at a crow than a man swear he loves me."
The point is that Shakespeare established the differences between these two couples early on, and continued it throughout the play.
The two couples are heavily contrasted in Shakespeare's 'Much Ado About Nothing'. This creates interest for the audience watching the play, and also tension.
Hero is a quiet, obedient, shy and feminine woman who marries the dominating Claudio. Claudio acts as if he has purchased Hero, and she is his asset. When Hero supposedly misbehaves, she is completely disowned, seen as nothing better than dirt, even with very little evidence to prove so. This relationship highlights a disturbing view of gender; a patriarchal family with a dominating male and an obedient female.
However, the relationship with Benedick and Beatrice is much more different. Beatrice is an outspoken, intelligent, witty and confident woman who possesses masculine characterisitcs, like a need for revenge. She enjoys exchanging insults with Benedick (which shows the instability of their relationship at the end of the play). Benedick ran around after Beatrice instead of Beatrice running around after him.