Combine 1 teaspoon salt, 1 teaspoon pepper, and next 4 ingredients in a medium bowl.
Sprinkle roast evenly with remaining 1/2 teaspoon salt and 1/2 teaspoon pepper. Brown roast on all sides in hot oil in a large Dutch oven over medium-high heat. Transfer roast to a 5 1/2-quart slow cooker. Pour tomato mixture over roast. Cover and cook on HIGH 5 to 6 hours or until meat shreds easily with a fork.
Remove roast from slow cooker, and cut into large chunks; keep warm.
Skim fat from juices in slow cooker. Mash 1 1/2 cans (about 2 3/4 cups) pinto beans; add to slow cooker, and stir until combined. Stir in black beans and remaining 1/2 can pinto beans. Add roast pieces back to slow cooker; cover and cook on HIGH an additional 20 to 25 minutes. Top each serving with jalapeño pepper slices, if desired.