How do I get one QB to take a great majority of the snaps (i.e. all snaps other than injury/blow out)? I set one QB in the depth chart but when I view "starters" in the roster window, the other QB shows as starter. They both generally get double digit passing attempts. I only mess with depth chart, play calling weights and game plans. Haven't dove into playbooks or formations. I have "backfield by committee" unchecked. What am I missing? Is my issue that I'm not adjusting formations?

You should be able to set one depth chart for each formation that has the one QB. There is an endurance component on by default that will swap out players if they get too tired, so you may need to tweak the settings in your league configuration if you want to keep the player in longer.