Mix flours in food processor; add salt and, with machine running, pour in one cup warm water. Process for about 30 seconds, then remove cover. Dough should be in a well-defined, barely sticky, easy-to-handle ball. If it is too dry, add warm water a tablespoon at a time and process for 5 to 10 seconds after each addition. If it is too wet, which is unlikely, add a tablespoon or two of flour and process briefly.
With floured hands, shape dough into a ball, cover with plastic and let rest for at least 30 minutes. (At this point, you may wrap dough tightly in plastic and refrigerate it for up to a day; bring to room temperature before proceeding.)
When ready to grill, pinch off a piece of dough and roll as thin as is practical. Dust lightly with flour to keep from sticking and cover with plastic or a damp cloth while you roll out remaining dough. (It is okay to overlap them a bit, but do not stack them.) Or just roll as you grill.
Grill chapatis until they start to blister, char and puff up a bit, about a minute or so. Turn and repeat. Serve immediately.