The "gunner" (as far as moveInGunner, gunner, assignAsGunner are concerned) always lives in the primary turret. On the blackhawk, this is the crew chief position. There can only ever be one gunner, as such, regardless of the number of turrets (blame BIS for not thinking that secondary turrets might be useful until ArmA). The secondary turret position actually has nothing to do with being a gunner so I think you'd need to assignAsCargo (or not assign) to get someone into that position. Anyway, I think that explains why your were having problems based on the order you assigned people as gunner: the second guy being made the gunner will have unassigned the first guy.