Prepare Chicken Fajita Filling:
Marinate the chicken in 1 tablespoon olive oil, lime juice, chili powder, cumin, oregano, garlic, salt and pepper in the fridge for at least 30 minutes.
Grill the chicken over medium-high heat until cooked, about 2-3 minutes per side. (Reserve the marinade.)
Heat 1 tablespoon olive oil in a pan. Sauté the onions until tender, about 5-7 minutes. Add the peppers and the remaining marinade and sauté until the peppers are tender, about 5-7 minutes.
Remove from heat and mix in the cilantro.
Heat another pan over medium heat.
Butter one side of each slice of bread, place one slice in the pan with buttered side down; top with the cheddar cheese, followed by the chicken fajita filling, the Monterey jack cheese, and finally the other slice of bread with buttered side up.
Grill until golden brown on both sides and the cheese is melted, about 2-4 minutes per side.
Serve with guacamole, salsa and sour cream for dipping.