If he chooses to turn away or go vertical, he ends up on the defensive against two craft instead of one, making that the worst possible choice.
I'm afraid this is untrue for TvT.
Whilst if he flew off in a random direction in a straight line, sure it'd be bad, but for the most part not attacking either craft IS the best choice, for several reasons;
No matter which craft you go after the way you describe you will be in a predictable pursuit curve, and easy pickings for even the greenest of rookie pilots.
If you go for the easy option you have a slight chance of killing the pilot who made it easy for you before the other guy catches up to you.
If you go for the pilot who is slotting in expecting to be behind you the other has to full turn AND head back AND he's probably used a fair bit of his burners already meaning he'll be delayed and this course of action will mitigate some incoming fire.
If you go completely defensive and play the long waiting game, assuming the enemy aren't in ships that can out turn yours, you will eventually find that no matter what they do if you fly well enough, regardless of their skill level they wont be able to hit you easily.
However, you control when you stop this manoeuvre, and you only need to do so for less than a second to deal significant damage in a decently setup ship.
The opponents can level this slightly by taking on the same tactic but that just means that the playingfield is totally level.
Having multiple vs singular isn't always an advantage in TvT either because if you have more targets to shoot at you're likely to pick up more points.