Mercutio actually is not present in Act 2, Scene 2, as that is the famous balcony scene involving only Romeo and Juliet. Therefore, it is a bit difficult to compare Mercutio and Romeo using this scene. However, they do appear together in Act 1, Scene 4, just before they go to Lord Capulet's house to crash the ball. In this scene, we can clearly see that they are each others' dramatic foil. A dramatic foil is a character whose traits are the exact opposite of another character's and is therefore used to "highlight or emphasize" the other's traits through contrast (Dr. Wheeler, "Literary Terms and Definitions: F").
One way in which Mercutio and Romeo are opposites is with respect to their views on love. While both view love in terms of sexuality, Romeo takes love deadly seriously, while Mercutio treats it as a joke.
We especially see how seriously Romeo takes love when we see how heartbroken he has allowed himself to become over Rosaline's rejection. We especially see his heartbreak in this scene when his response to Benvolio's and Mercutio's attempt to persuade him to crash the ball is that he feels far too heavy from his heartache to dance, as we see in his lines:
I am too sore enpierced with this shaft
To soar with [Cupid's] light feathers; and so bound
I cannot bound a pitch above dull woe.
Under love's heavy burden do I sink. (I.v.20-23)
In contrast, rather than taking love as seriously as Romeo, Mercutio treats it as a joke, as we see through his mocking responses to Romeo, such as telling Romeo to "[b]orrow Cupid's wings" or telling him to be rough with love back for being rough with him.
In addition, while we know that Romeo equates love with sexuality and physical attraction, as we see in the very first scene through his comments about Rosaline's beauty and her vow of chastity, another difference between he and Mercutio is that Mercutio merely sees love as a sexual joke. We can especially see Mercutio treating love as a sexual joke through his lewd comments found in later scenes, but even in this one we see some sexual innuendos that show us Mercutio's flippant feelings about love. For example, these lines can be translated as being a sexual innuendo: "If love be rough with you, be rough with love. / Prick love for pricking, and you beat love down" (28-29).