You can award multiple attacks but I’d stick with narration of the frenzied effect of having someone “dead to rights”.
As said, throw multiple boni(bonusususes/mali(malusususes) for both OB and DB(Static/Helpless target) in this kinda scenario.
Heck, maybe forego the multiple attacks and reward multiple crits in descending order of what they get from their initial one?
(After all the boni/mali are tallied, the initial attack roll was poor, still, and *only* gives a ‘15C’ Crit on the table lookup.
This means the attack ends up giving additional ‘B’ and ‘A’ criticals, to reflect the law of diminishing, yet deadly, returns!)
It opens up a tonne more special rule scenarios your players will try to finagle from you, but you have to know your audience at the same time.
If they won’t try to leverage this one special move into other areas or you see they try to apply it to every. single. situation, then you have a great bunch of players!
G’luck in your game.
Is your assassin any good?
Then give them +55 for flank/rear/surprise or the helpless target bonus, or something similar, and no DB for the target. When they easily get an E crit, use their 20+ ranks of ambush to give an excellent chance of a kill (any result of 46+ should do it, lower if they have enough Ambush ranks).
Narrate multiple stabs, with the fatal crit being the most deadly one.
If they can't manage the kill ... looks like the target noticed them at the last moment and shrugged or moved away.