Realizing that Good-Cop/Bad-Cop is a tactic meant to Divide Targets resources. When you are making a decision and trying to predict your outcomes. Creating two sets of outcomes based on two different personalities is harder than creating a set of outcomes for One personality - either good or bad. You can better foresee the outcomes if you're dealing with one Speaker/Target/Claimant/Customer/Client etc...
So as a GM I would require some Social skills to employ "Divide and Conquer" or "Flanking Attacks" or "Good Cop and bad cop". A Clueless person about how the tactic works more likely will waste the tactic. At least one of the two is skilled.
Next is that I will use Advantage or Boon or apply the Inverse modifier: Bane or Disadvantage to the person who is under this circumstance.
Imagine yourself with your friend who you know well enough to know his/her tells, strengths and weaknesses. You're in a negotiation and advance a point of discussion that complicates the opposing party. it doesn't have to be a really good argument, even a bad argument works fine, as long as it Divides the Targets attention then it prevents the target from successfully arguing his case by building up credibility or strong arguments.
This can be reversed, a skilled negotiator will use every negative point against the two people who are arguing against him. Dictating the pace and managing from an advantage position (the burden of proof is on the other party; one party has to make more effort per argument).
The same Biomechanics fighting multiple Opponents requires can be said about the Speech and Conversational Pacing mechanics - you have to be an expert in taking the weaknesses and flaws of one of the opponents to give the attackers obstacles.