In a large bowl or the bowl of your slow cooker, toss the shredded cheeses with cornstarch until evenly combined. Add the cheeses to the slow cooker and pour over the evaporated milk. Stir in the remaining ingredients until evenly distributed.
Cover and cook on low in the slow cooker for 90 minutes.
Set the slow cooker to warm, uncover, and stir until smooth. It will be thin and grainy at first but will come together as you continue to stir.