Give each driver a destination they can reach independently
Share the address, entrance description, and a time window. If one driver misses a turn, they should be able to choose a safe route to the destination without trying to catch the other car. Decide in advance what happens if the usual car park or entrance is unavailable.
Map passengers and belongings for both legs
Suppose Lina rides out in the first car but returns in the second. Her coat and return ticket need to move with her, and the second car must actually have a suitable seat. Assign one adult to confirm the swap with both drivers, including any passenger who cannot manage the handoff alone.
Handle a split arrival
If a passenger changes cars at a stop, confirm which driver has them before either car moves. Give both drivers the full written destination and a meeting window. If the cars separate, each can travel safely without trying to follow the other through traffic. Choose a fallback meeting place for a closed entrance or full car park.
If the picnic is in the later car, the earlier group can take a short walk but should not tell everyone lunch will start immediately. Check who has the item that unlocks the next activity, whether it is the entry ticket, folding table or keys. Complete the same check before the return journeys; the outbound packing list no longer proves where everything is.