First of all, you have to decide upon your target priorities. View somebody else's photos of Yellowstone, and decide what it is that you intend to shoot. As an example, if you are going after scenic panoramas, then the 18-55mm lens should be sufficient. You probably want to be able to drop a circular polarizing filter on that to help out the sky contrast. If you are going after wildflowers in close, then the same 18-55mm lens will work, or else shift over to the 70-200mm. You need to check out the minimum focus distance for that last lens. Also, get about 2-3 yards of white cotton cloth and about 4-5 metal coathangers. Straighten out the coathangers and use the metal to hold the white cotton upright to make a white tube to surround the wildflowers. It will give you a more diffuse light. Direct sunlight tends to be too contrasty. As the lenses get longer and longer, you tend not to use so many filters. I have two longer lenses, and neither of them have seen any filter in six years of use. A graduated neutral density filter can be handy if you have super bright skies with dark foregrounds. The GND filter "holds back" the brightness by a couple of stops, and that allows you to meter more accurately and get results.
If you are going after wildlife, you may want a longer lens.
There are some good photo guide books on Yellowstone, so you might want to read one of those before you go. Where to shoot and when to shoot. Before my first Yellowstone trip, I was able to plan a shot list, and I optimized it so that I would not be driving back and forth aimlessly.