There's no reason not to allow this mechanically, or even any realistic reason why you couldnt shoot more than once if you were close enough. That's the key.
Most of the time, against well-trained, observant targets, the sniper is going to be so far away that Far Shot and Dead Aim are much more valuable.
On the other hand, if a sniper is within 100 feet of a target (very close) then there's no reason not to shoot more than once.
Official Viet Nam statistics for snipers are never one shot per kill. Sometimes the numbers of 6 rounds per kill, sometimes as low as 1.4 rounds per kill. But you can see that "one shot one kill" is more a goal than a reality.
Also, a common sniper tactic when "decapitating" a unit is to identify an officer, shoot him in the gut, then shoot him in the jaw to prevent him giving orders but attempting to leave him alive (so his men will expose themselves to enemy fire and attempt to save him).
In this situation, the gut shot-jaw shot combo might very well be considered a double tap in game terms.
Chuck