Adjust oven rack to upper-middle position and heat over to 500 degrees. Set wire rack in rimmed baking sheet. Cook bacon in 12-inch non-stick skillet over medium heat until crispy, 7 to 9 minutes. Transfer to paper towel-lined plate. When bacon is cool enough to handle, chop fine and set aside.
Season jalapeños with salt and place cut side down on wire rack. Bake until just beginning to soften, about 5 minutes. Remove jalapeños from over and reduce oven temperature to 450 degrees. When cool enough to handle, flip jalapeños to cut side up.
Mix cheddar, Monterey Jack, cream cheese, scallions, cilantro, panko, egg yolk, line juice, cumin and bacon together. Divide cheese mixture among jalapeños, pressing into cavities. Back until jalapeños are tender and filling is lightly browned, 9 to 11 minutes. Let cool for 5 minutes. Serve.
To make ahead
The filled and unbaked jalapeños can be covered and refrigerated for up to 1 day. Add 3 minutes to baking time.