Starting from Happy Isles Trailhead and hiking to the top of Half Dome, going back down the spur trail, then hiking up to Clouds Rest and then hiking to Sunrise Camp, is about 22.2 miles, with a 5,900 ft elevation gain. That is one pretty tough day. My advice is if you want to climb both, start at Happy Isles and hike to the top of Half Dome, and hike just past the spur trail to Clouds Rest on the JMT and camp just north along Sunrise Creek. Then hike up to Clouds Rest and then up Sunrise Camp. However, if you are going the opposite direction, meaning starting at Sunrise Camp and finishing at Happy Isles Trailhead, then doing both in one day is far more possible to do both. But still a tough day. I have day hiked out of Tenya Lake and hiked over both in one day, but you wait for a couple of hours to climb the cables up to Half Dome. Good Luck.
Trihiker