You’ll need to send heroes in pairs to events as often as possible. If they successfully complete a mission, their synergy level will increase. Once the first level is reached, you’ll see a purple window above their avatars saying “Synergy+”, and upon reaching the maximum level, a duo banner will appear.
The duo synergies are:
• Sonar / Malenova
• Flambae / Prism
• Invisigal / Golem
• Coupé / Punch Up