I agree with Diggles, it should just be automatic.
It will be tough at first but people will learn to be more careful. 90% of TW are completely avoidable, it's all just carelessness, people recklessly swinging wide with teammates nearby (learn 2 overhead/stab in those situations, don't just keep spamming left click, hold back a second and line up your strike when someone is spinning off targets), people just being greedy and attacking anyways when there's basically a teammate between them and the enemy, or ranged people shooting into melee at terrible angles.
The only TW that's not avoidable is when 2 people swing at an enemy at same time, and the first one kills him so the second continues to travel and hits teammate. If corpses retained their physical properties for a few seconds that could be solved, cuz right now we all seem to be some kind of jedi ghosts.