The short answer
You can, but we do not recommend it. Midday at Merzouga in July and August can pass 45°C, many of the better camps close or cut back, and the camel ride shrinks to a short dash at dawn. If your dates are fixed in high summer, do the Agafay at sunset instead, or head for the coast and the Atlas.
In more detail
Every desert guide in our files says the same thing: October to April is the Sahara season, and July and August are the months to avoid. The heat is not just uncomfortable; sitting out in open sun in the afternoon without shade and water is a real risk. The long drive over the Tizi n'Tichka is tiring in the heat too.
If you go anyway, shape the trip around the cool hours. Ride out at first light, keep everything outdoors before about 9 in the morning and after 5 in the afternoon, and choose a camp with cooled tents and a generator. Expect warm nights rather than cold ones.
The better summer plan from Marrakech is to use what is close. The Agafay is lovely for an evening once the sun is low, and you are back at your riad pool within the hour. Essaouira on the coast stays breezy, the Ourika valley and the High Atlas run cooler as you climb, and summer is the season for trekking from Imlil.