Would this be an implication that they live together in America, or just simply that they both happen to live in America?
हैं (with the dot which indicates that its vowel must be nasalized) is used with third-person plural subjects (like जूलिया और पीटर). In addition, it is also used when the subject is हम (we) or आप (formal form of 'you').
है is used with third-person singular subjects and तू (very informal form of 'you').
There is also हूँ which is used with मैं ('I') and हो which is used with तुम (informal form of 'you').