Question

Which two elements combine to make most of the common rock forming minerals in the crust?

Answer 1

Those two elements are silicon and oxygen.
Silicon and oxygen together form most of the silicates which can be found in Earth's crust. They appear on other planets too, as well as moons and asteroids. The combination of silicon and oxygen creates many types of minerals, such as olivine, pyroxene, amphiboles, quartz, etc.