This and The City are my favorite stand alone versions of Carcassonne. Using only the road as the hard join points of a tile make for some very interesting strategies. Now on to your question, lol.
From the rule booklet: The player scores 2 point per tile for 1 uncompleted tower (at game end).
I tend to be very "lawyer-ish" about rules. So my opinion is, you can only use one wall tile per feature.
My reasoning is the following: When you have scored a feature the last step is to remove the meeple from the feature. So in this case you present the wall tile, you score the uncompleted feature, you remove the meeple. That to me suggests you can't use two wall tiles on the same feature.
But then again I am not Not HiG or Reiner Knizia. So I would play it how you desire. Just make sure everyone knows the rule you want to use.