No, non-artillery has to shoot the element most directly in front (not necessarily the closest) (check the exact rule wording). An element is not allowed to ignore its own target and aid against somebody else's target.
When multiple elements all have to shoot the same target, you can choose which is the primary and which will aid (except if the enemy shoots back, then the one that is shot back at must be the primary). (remember that 2nd rank Bows can only aid, they can't be a primary)
If you manage to saturate a target (i.e. more than 3 elements would shoot at it), the 4th element will shoot its 2nd priority target instead and you can choose which of your shooters will be this 4th element (or 5th etc).