Sometimes you can get it to go to that spot next to the sieged city, but it's not easy. Your options are: select siege city, then it will move in and go to a tile adjacent to the city, but it might not be the one you wanted. Or you can zoom the map out and try to select that tile that way (select your army to go to that tile I should say). Sometimes that works. It probably has something to do with the siege graphics we have, but I'm not certain. It doesn't seem to be a problem too often, but I understand what you are saying, and it does pose a problem sometimes. Try one of these two things I mentioned though - I'm sure your second army can make it there, it just might take some work to get it right on the right spot.