If Kirk needed to serve 1/2 of a can of cat food each day when he had one cat, then he needs to serve 1 can of cat food per day to feed two cats.
Since Kirk now needs to serve a total of 2/3 of a can each day, he needs an extra 1/3 of a can of cat food each day to feed the second cat.
Therefore, Kirk needs an extra 1/3 can of cat food each day to feed the second cat.