This is a situation I've seen and been in once or twice, and was curious on what other people's opinions are for their course of action.
Scenario:
You're cruising along at altitude on the way to a fight, when suddenly you see below you a friendly being chased by two bad guys. The enemy fighters are outside of gun range but closing slowly, and are flying as a wing-pair (within D100-200 of each other), with one leading and the other trailing behind and off to one side to keep his own line of fire clear. There are no other cons in the vicinity, friendly or hostile. Let's say you're within a friendly dar circle and you can confirm there's no enemy lurking outside visual range. You happen to be flying without tracers turned on.
You radio the friendly you see him, and to continue pulling the bad guys out straight, then after neatly maneuvering for position roll in (so no, this is NOT a "Do I help my buddy?" morality call). You carefully manage your speed to ensure you close into gun range before the enemy can do the same to your friend, without overtaking them too quickly to get a good shot. You settle in behind them and watch the range counter click down. They're completely fixated and don't see you coming. Your finger is on the trigger and the counter on the trailing bandit hits your convergence range, however you still have a clear shot at BOTH targets.
Question:
Which do you shoot first?
A) Trailing Con - If you hit him solid enough and even get a pilot kill you may be able to knock him out before he can even warn his lead plane that he's been hit. You'll also have more time to take aim on the leader. The downside is, if the leader has been able to close into gun range on your friendly you may not have time to hit him before he can open up on his own target.
B) Leading Con - He's closest to the friendly and in the best position to fire. Additionally, if the wingman has his leader in view you'd have the twisted satisfaction of scaring the HELL out of him as his boss suddenly bursts into flames and making him realize that they've just screwed up very, VERY badly. However you would then have less time to correct your aim for the trailing bandit, and he may be able to break, out of your line of fire and make you work a little harder for the second kill.