Now I see your situation very well and I follow your logic. I'm not a dirt man, but I think that cutting the tile (and packing in clay) in one place , but wider, might be better than cutting it twice. Either way I bet you'll have good results assuming that there are no unseen paths for water to travel in/through the dam besides the tile. I would also get the trench closest to the pond as closed to the pond as safety will allow. Tile runs may also contain soils that are not clay from the original backfilling, so I would be extra observant of the soils coming out of the trench/es and make sure you go deep enough to get back to good clay.
I don't think adding bentonite can hurt anything, but I don't know much about bentonite.
