Membrane
Two Physarum swarms forage in two adjacent fields split by a permeable membrane. How much each can hear the other across it decides whether their two voices lock into call-and-response or drift into counterpoint.
Start the two swarms. A seeded autopilot sweeps the membrane permeability from closed to open and back and plants food, so you can watch and hear the coupling evolve hands-off — then take the keyboard.
Keyboard only: ←/→ (or [ ]) open/close the membrane · 1–8 plant food · Space switches the field you feed.