I would simply group it into one. Wikipedia even groups Zion and Bryce closely. From my recollection Bryce felt way different than Grand Canyon and was way more accessible. Pretty sure this is one of the best NPs I visited.
How about Zion and Bryce Canyon (and perhaps Cedar Breaks NM) all together as they are all (more or less) adjacent to each other? Arches NP is far to the Northeast, about 5 hours drive from Zion.
Perhaps a name like Colorado Plateau National Parks: Zion & Bryce Canyon? Here is the NPS website for Utah National Parks
, which shows the exact locations.
"The Colorado Plateau has the greatest concentration of U.S. National Park Service (NPS) units in the country outside the Washington, DC metropolitan area. Among its nine national parks are Grand Canyon, Zion, Bryce Canyon, Capitol Reef, Canyonlands, Arches, Mesa Verde, and Petrified Forest. Among its 18 national monuments are Bears Ears, Cesar Breaks, Rainbow Bridge, Dinosaur, Hovenweep, Wupatki, Sunset Crater Volcano, Grand Staircase-Escalante, Natural Bridges, Canyons of the Ancients, Chaco Culture National Historical Park and the Colorado National Monument." - Wikipedia