Combine Kahlua and butter in a small saucepan, bring to a boil. Boil 4 minutes, stirring often. Remove from heat, and let cool slightly. Slice tenderloins crosswise at 1 1/2-inch intervals, cutting to, but not through, bottom. Sprinkle tenderloins with lemon-pepper seasoning and garlic salt; place in a large shallow dish or large heavy-duty, zip-top plastic bag. Pour half of the Kahlua mixture over tenderloins. Cover dish or seal bag securely and marinate in refrigerator 4 hours, turning occasionally. Cover and chill remaining half of Kahlua mixture.
Remove tenderloins from marinade, discarding marinade. Grill, covered, over medium-hot coals (350 - 400 degrees) 18 - 20 minutes or until a meat thermometer inserted in thickest portion of tenderloin registers 160 degrees, turning once and basting occasionally with remaining half of Kahlua mixture. Cover and let stand 10 minutes before slicing.