def span_position_distance_bilocal(signature1, signature2):
    """Special span position distance function for intersepersed duplication signatures"""
    distance_normalizer = signature1[3]
    span1 = signature1[1] - signature1[0]
    span2 = signature2[1] - signature2[0]
    source_center1 = (signature1[0] + signature1[1]) // 2
    source_center2 = (signature2[0] + signature2[1]) // 2
    position_distance_source = abs(source_center1 - source_center2
        ) / distance_normalizer
    position_distance_destination = abs(signature1[2] - signature2[2]
        ) / distance_normalizer
    span_distance = abs(span1 - span2) / max(span1, span2)
    return (position_distance_source + position_distance_destination +
        span_distance)
