In a large bowl, combine the rinsed cannellini beans, chickpeas, diced red onion, minced jalapeño, and chopped cilantro. Toss gently to mix.
In a small bowl, whisk together the fresh lime juice, olive oil, ground cumin, salt, and black pepper until well combined.
Pour the marinade over the bean mixture and stir gently to coat everything evenly. Cover and refrigerate for at least 15 minutes to allow flavors to meld.
Place a tostada shell on each plate. Spoon the marinated bean salad generously over the tostada. Top with sliced avocado and crumbled cotija cheese if desired. Serve immediately.