These events are time-ordered:
- Attacker's Attack Roll
- Your Sacrificial Defense Roll
- Target's Defense Roll
The defender rolls only when failure to do so means no further chances to avoid being hit. He doesn't roll if an attack misses and he doesn't roll if a sacrificial defense succeeds – but once he
does roll, the assumption is that the blow has made it out of the gate, past any intervening figures, and to his doorstep. Retroactively deciding to try a sacrificial defense for a friend who's in the process of defending might well cause more harm than good: "Sorry I stabbed you in the face – I put my sword there because I saw you hadn't ducked that blow enough and it was going to hit you in the head!"
Being able to declare a "safe" sacrificial defense after the point of no return would be a huge special benefit. I'd call it a technique with the usual -1 for an ordinary-sized special benefit (defending someone else) plus an extra
-2 for messing with the flow of time. This would be Hard, cinematic, and probably only an option for the same sorts of characters who can invoke Bullet Time.