Well, here's the problem: I'm not sure they both are foils, primarily. Beatrice is one of the major comic engines of the play. She's so witty! Hero is a much simpler character. She's so idealized that she is almost flat. A foil plays opposite the main character, and interacts with him/her to show the main character's traits better.
I guess Beatrice could be a foil for Benedict; the two of them are foils to each other for their wit, the way they both stand apart from love, and the way their egos let them be manipulated.
Hero could be a foil to Beatrice more easily. Where Beatrice is lively, witty, and divorced from love, drawing attention through her wit, Hero is quiet and virtuous, and draws attention through her modesty. They are thus foils to one another.