In a standing mixer fitted with the dough hook/kneading attachment, stir together flours and salt on low speed. Add the oil and enough warm water so that the dough comes together in a smooth ball. It is right when it sticks a bit to the bottom of the bowl but releases from the sides. (If you make it too sticky, add more flour a tablespoon at a time). Once the dough forms, reduce mixer speed to low to knead for 10 minutes.
Cover bowl and let dough rest 20 minutes at room temperature (or up to three days in the fridge) . Flour a clean work surface, then pinch of pieces of dough about the size of a golf ball. Preheat a cast iron skillet over high heat for 5 minutes. Roll chapatis into flat circles with a rolling pin on a floured surface (flouring the surface well is key to making proper circles, if you're inclined).
When skillet is very hot, add a tortilla/chapati. Cook until bottom has lost most of its sheen, is a bit bubbly and perhaps lightly charred in a few spots (30-90 seconds, depending on how hot your skillet is), then flip and cook about 15 seconds more. You want them to appear slightly under-done (still have a bit of sheen) and remain soft. As they cook, transfer to a plate and cover with a clean towel.