You don't realize just how big Yellowstone National Park is. You will waste so much time driving to and through the park that you won't have time for even short hikes. Then trying to drive down to the Tetons for a day! Way too much time driving instead of looking. I highly recommend you consider staying in or immediately outside Yellowstone for at least three days then go down to Tetons National Park for a minimum of two days. Staying between the parks to see them both can definitely be done, but not if you want to really experience the parks.