Introduce the activity, and clarify, that they are going to take turns arguing both sides of an issue. One person speaks at a time, and the facilitator will call on people back and forth to keep the debate moving forward. Don't be shy about calling on someone who doesn't have their hand raised, and make sure to not let a few people dominate the debate. Also make sure to explain that regardless of how each person personally feels about the issue up for debate, for the next 20 minutes, they are to assume the role of one side, then they will have to switch and argue the other side with their best persuasive arguments.
Assign one group to one side of the issue (this will be the "pro" side - who will argue in favor) and
assing the other group the other side of the issue (the "con" side - who will argue in opposition).
After about 10 min (or just when they seem to be getting really entrenched in their sides) stop the debate and say "Switch!" and now they each have to take the opposite side of the issue and build on the arguments already made.