We're wondering if Kanab to Saint George is a doable distance in one day? I've been using mapmyride and google maps to look up distance/elevation but since you can't bike through the tunnel it gets messed up for this part of our trip.
Only you can say if the distance is suitable for you to do in a day.
On the tunnel issue, I will say that I have never found hitching a ride to be too difficult where it is necessary, especially in national parks. It might be a little more difficult for two, but I can't imagine it would be a huge delay. I have had to hitch a ride during a bike tour a couple times and been with others who needed to quite a few more times and the wait has never been long. In the desert it seems folks are especially likely to stop. In places where there are very few cars more of them will stop.
On backpacking trips in national parks I have usually had the first, second, or third car give me a lift. It is a bit harder with a bike, but still I doubt it will be a big deal. If in doubt get a very early start that day.
It looks like there is a visitor center near the tunnel, asking around there will probably score you a ride in quick order.
None of this is based on any personal experience with that particular route though so take it for what it is worth with that in mind.