I had uploaded an example of the model. In this model, trolley 2(I change the design from Queue to Trolley design) had 20 flow item(cartons) and in trolley 1(The same as trolley 2), is the preparation trolley. I would like the operator to prepare 20 cartons in trolley 1 and when trolley 2 had utilize 20 cartons, the operator would push trolley 1 to where trolley 2 is and trolley 2 would be pushed to where trolley 1 is.