The main reason the French poilu carried two Bidons was one bottle typically contained plain water and the other either wine (pinard) mixed with water, coffee or a tafia spirit mix.
Bidon Modèle 1877 (Canteen Model 1877): These were came in both 1 and 2 litres versions which featured two spouts, a large and small, which allowed the soldier to have two drinking options available to them, these were stopped with a cork on the large spout and a wooden plug on the smaller spout, both of these were secured to the canteen itself with string. These canteens were usually covered horizon blue wool by 1917. The double-sized 2-liter canteen was originally reserved for issue to troops in Africa prior to the war, but the difficulty of supplying clean water to men in close combat was considerable and it prompted the introduction of the double-sized water bottle to all troops in the summer of 1915.
Quart Réglementaire Modèle 1852 (Statutory Quarter Cup Model 1852): The quart was used for drinking the issued pinard or tafia spirit mix, they were made of tin which was usually hung from one or both of strings or the sling on the canteen.