Honestly if you see this sort of situation developing in a game you shouldn't be aiming for full teamfights, but forcing them to either separate or commit to a situation (ie push separate lanes, baron while pushing, etc). If they split up you win the fight where you, the poppy, is, and the other pushers back soon enough to escape. If they commit to one, that side escapes while the other continues pushing. A couple well-placed wards can serve as the catalyst for this tactic.