Another way to attach water bottle(s) is to use a Minoura handlebar mount. Mount the bottle facing inward on bar (toward stoker). To facilitate drinking, without removing the bottle, you can pop out the plastic bottle pop-up spout and insert an appropriately sized diameter flexible plastic tube, available from most local hardware/home improvement stores or you even local pet shop that handles aquariums/fish.
Stoker Kay uses a custom mount, dead center on her stoker bar stemclamp. Works great for her.
Pilot uses one mounted on his bars with rigid plastic tube. No fussing/mussing riding one-handed to drink.
Stoker also prefers the round c/f handrests over the pseudo-brake levers on her bars.
Stoker also prefers not to use a suspension seatpost. It's up to the pilot to call out the bumps/road hazards!