At hotkey F2, there are two stone stockpiles next to a track stop, with the track stop built to dump west (into a third stockpile). There are some minecarts sitting in the carpentry shop. Create a hauling route with a stop that takes from the two 2x2 stone stockpiles, assign a vehicle, and set the stop to take both kinds of stone with no departure conditions.

After a few moments, dwarves will start moving stones from ONE of the 2x2 stockpiles into the minecart, but not from the other stockpile.

If you do all that, save, and reload the game, they will start moving stones from BOTH stockpiles.