Okay, well the lasting injury rules state that the opponent of the injured fighter rolls. But yes, the idea of Maiming is that you, the opponent, roll twice and pick the result.
However, this toxin is just terrible. It will only trigger if you roll an OoA on the injury dice; killing with a Coup de Grace, or a Serious Injury that gets worse in the recovery phase, will not trigger it. So you have at best a 1/6 chance of that coming off.
And since you need to roll a 5 or a 6 on the first d6 of the D66 to actually cause them serious mischief, even if you trigger it you've only got a 55% chance of doing lasting damage.
The cheesy alchemy combos are all in the Gaseous Ammo, but the Nightshade is pretty close range and a Choke Gas Grenade Launcher is a prime 'Click...' target