I like to carry two bottles because if I carry only one, then I get thirsty. Actually, if I carry only one, then the bottle gets lonely.

If I carry two, my bike is more aero as well because I only drink from the bike on the down tube, not the seat tube. So the seat tube bottle compensates for its weight by decreasing drag and making me faster, even if my bike is heavier. Unless the grades increase past 8%.