Koopmans was the last name of a dude who gave a rat a second stomach, and fed it through a tube, while the original stomach was fed in the usual way. He tied a piece of string around the output tube from the new stomach to act as a pyloric shyncter (which controls how much and when food goes from the stomach into the small intestine). He also attatched to the new stomach nerves and blood vessels that went to (and came from) the same locations as blood vessels from the other gut did. The rat did not consume twice as much food.
What happens is that you’re stomach produces a peptide known as Cholesystokinin (CCK) that latches onto the CCK-A receptors on the nerves surrounding your stomach, which transmit information to your brainstem, which is then passes the information to neurons that eventually synapse on the Ventro-Medial area of your hypothalamus, which tells you that the body has enough substance for your insulin (which had been released from the pancreas to make you hungry about ten minutes before you started eating) to handle right now. The name of the rat he used will forever remain a mystery.