Given the particulars of the setting and scenario, there's no reason at all why we can't have more than one sniper - the idea is that everyone is isolated from their respective chains of command and organizations, rather than all being part of the same organization or group, and come together through random events rather than by being placed together - banding together for shared goals and protection, rather than being ordered to do so. 

While I realize that doesn't reflect the 'authenticity' part regarding military 'stuff', it does make it more interesting and more survival-based, and is an alternative to let you (i.e. the players) play people from different groups and backgrounds.