If I were you, I would take the 1DX and the Canon 600 f4, the 70-200, and the 35 f1.4. Use the 35 to take landscapes and pictures of your wife, use the other two for pictures of other animals. Take a pocketable point-and-shoot with which you might ask someone to take a picture of you and your wife together.
I would take the 1DX for the combination of full-frame and high fps. I don't really see why more than one DSLR would be necessary... unless this was purely a photography expedition and you were leaving your wife at home.
I'm not a professional photographer, but thanks to living in northern Utah, and my parents' timeshare in Jackson, I have been fortunate to go to Yellowstone almost every year for the last 20.
Forget the gear and figure out what the purpose and priorities of the trip are. This is one of the the last chances you and your wife will have to be together alone for at least 10-20 years. Do you really want to spend it with your eyes glued to a viewfinder while your pregnant wife sits there being devoured by mosquitoes?
Take plenty of pictures... but use this trip as an enjoyable experience for both you and your wife, use it to familiarize yourself with YNP, then return another time as the photographer - who sits in one place for hours, waiting to be eaten by wolves.
With my family, we usually spend a day in Yellowstone. Each year, we choose which half of a loop to see, of the two loops, and that's about all there is time for, in one day, if you're not staying in the park.
The maximum speed limit is 45mph, but you might average out to 20mph, if the bison herds are near the roads.
Sometimes we see a lot, and sometimes we don't see much. I once backpacked through the backcountry with my scout troop for five days, and didn't see anything. This time of year, you will probably see a lot of animals, but also lots of overcast, so don't count on astrophotography.
Personally, I would either invest in a few more CF cards or take the laptop.
Pack a lunch, snacks, drinks, hand sanitizer for the outhouse, and good tunes. Stay on the designated trails or die!