Conveyor belt looping


I am new to Blockly. I am doing a project which involves lopping. One dobot should pick cubes from pallet and place to conveyor belt and the other dobot should place back to next pallet. Then now the second dobot should picks and places to the conveyor belt while the first dobot should place back to the original position.
How should I programme the loop? I know when I change the stepper motor speed to negative it moves in reverse but I am really confused how should I programme everything. Any kind of help is appreciated!