Cook chorizo in large skillet over medium-high 6-8 minutes until browned and fully cooked, crumbling with spoon. Cook onion and garlic another 3 minutes until softened; drain excess grease and transfer to slow cooker.
Mix in chiles, Monterey Jack, Oaxaca, cream cheese, heavy cream, and cumin.
Cover; cook on low 1½ to 2 hours, stirring every 30 minutes, until cheese melted and dip smooth and creamy.
Add cilantro; switch to warm.
Top with tomatoes and optional extra cilantro; serve from cooker with plenty of tortilla chips.