Claudio and Benedick can be regarded as foils to one another for several reasons. First, Claudio is the young soldier ("my lord Lackbeard"), while Benedick is a more seasoned veteran of battles. Claudio is also the eager one concerning love and women - he falls head over heels for Hero, while Benedick scoffs at the idea of giving his heart to only one woman. However, also on the subject of love, we find that Claudio, despite his sudden passionate regard for Hero, is also quite untrusting of Hero - he believes anything Don John, a known villain, tells him about Hero's virtue. On the other hand, Benedick remains loyal and steadfast to Beatrice, once he is convinced of her love for him. He even leaves his service to Don Pedro after Hero's disgrace, showing his loyal dedication to Beatrice and her family.
Give Much Ado About Nothing several close reads - I'm sure you'll find even more ways in which Claudio and Benedick are foils to one another! Good luck!