As for how you place the tile you're attacking with, you're not required to put it on the board at all. It does not count toward that attack's bonus. Placing it overlapping the two stacks involved in battle is just to make it easier for both parties to be aware of which stacks of tiles are being contested.
If you have any other questions, let me know.
So just to be clear if I have a stack of two tiles adjacent to an enemy's stack of three tiles, during my turn I can place a tile overlapping the two stacks and the defender would have a +1 advantage?
I just picked this up a couple of days back and am looking forward to introducing it to my gaming group next month.
Thanks in advance,