they are "hollowed out"; there isn't a wall in-between. like this: you have a 10 foot hose and make a vertical slice from one end of the hose just up to the 3 foot mark. you then take the sliced portion, fold it over back on itself and sew it, so it now is like a baggie. Or like this: you take your left sweater sleeve, slice it lengthwise from the elbow to the hand. Do the same thing on the right side. Now sew the left open part to the right open part. not sure if that even made any sense! but yeah - "hollowed" out!
it helps to see a diagram. even though there is a "line" down the center, that just shows where the 2 pieces were sewn together to make the "holding tank"
http://j-pouch.org/illustratedpouch/