Putting the mid horn in front of the basshorn is OK, as the low frequencies from the basshorn will find their way around obstacles. From a time align standpoint it is best to have the actual distance from the driver diaphraghms to the listening position the same, which usually would mean placing the mid horn in front of the basshorn would be a good thing, as long as it doesn't interfere with the room furnishings or exacerbate the WAF.