I'm adjacent to both Tower hexes for already severals turns.
On my turn, I use my first Tower ability then I decide to move my single settlement that was adjacent to the other Tower hex, so that I'm no more adjacent to both Tower hexes. Thus, I must immediately discard in the box one of my Tower tiles.
Which Tower tile do I discard : the one already used or the one ready-to-use ?
Whichever one you want. You don't have to keep track of which tile meant which Tower; when you lose one you can choose to have it be the one you used already this turn or not.