Who are two characters in Act One who are foils for each other, apart from Mercutio and Romeo?
Since the question asks for "two other characters," then the Nurse and Lady Capulet act as foils to each other. Certainly, they are in contrast to one another as Lady Capulet pointedly asks her daughter to consider Paris as a husband, speaking in elevated, metaphoric language, comparing him to a "volume" and speaking of the marriage and the financial benefits to Juliet and to the Capulet family:
That book in many's eyes doth share the glory.
That in gold clasps locks in the golden story;
So shall you share all that he doth possess,
By having him, making yourself no less. (1.3)
In contrast to the business-like seriousness of Lady Capulet, who is direct in her speech, the loquacious Nurse, is much earthier in her urgings,
An honour! were not I thine only nurse,
I would say though hadst suck'd wisdom from they teat. (1.3.)
While Lady Capulet is serious and refined in her speech, Nurse is loving, light-hearted and rather ribald in hers. Nevertheless, she is affectionate, but there is little evidence of this caring from Lady Capulet; instead there is a certain tension between her and Juliet.
Romeo and Juliet is rife with foils, in my opinion. Consider, for example, Paris--Juliet's official (approved) suitor--and Romeo. Paris goes to Capulet to discuss his honorable designs upon his daughter. Paris is older and established and a gentleman, a good man to match with a wealthy, eligible daughter (in the eyes of Capulet, even though he wants Juliet to age a bit more before marrying her off, as she's only fourteen). Paris takes the proper path: he speaks with the father first to make arrangements and obtain permission.
Meanwhile, Romeo does not. Of course, Romeo cannot, thanks to the feud between their families. Considering that this play is primarily about the disastrousness of the feud itself (more than it is about young love--the doomed love itself is merely a symptom of the bad blood between the families, the play's main theme), the way Romeo must go about wooing Juliet is not and cannot be formal and proper.
When we see Paris discussing his plans with Capulet, we have the foil (of propriety) against which we see the damaged and ultimately doomed love of Romeo.
Foil characters need to have some important things in common. When they act differently, that serves to highlight something important about their nature. Benvolio is a young man and kinsman to the Montague family. Tybalt is of a similar age and cousin to Juliet, one of the Capulet family. So I think you could use Benvolio and Tybalt as foil characters.
When the fight breaks out in Act I (the one that starts with the "biting of thumbs"), both Benvolio and Tybalt come on the scene. Benvolio's first thought is to have the men part their swords, to make peace. But when Tybalt enters, he speaks of hating peace. He says, "What, drawn, and talk of peace! I hate the word,
As I hate hell, all Montagues, and thee:
Have at thee, coward!" and intensifies the fight which would eventually lead to the Prince's intervention.
This readiness for violence is what also leads to Tybalt's eventual death.