I killed both snipers, one much later in the playthrough though.
Still, achievement did not trigger. One sniper fell to his death.
Of course, now they are not re-spawning and achievement is impossible to achieve.
EDIT: Now on NG+ I realize there are snipers also in Jericho City. How many or how to get to them I do not know. So the achievement is probably not bugged. Sorry for mis-reporting.

last edited by n808