Friends in Berlin clued me in to this product by bikepacking company Cyclewerks in New Zealand that gets around the pesky problem of how best to carry water bottles when using a full frame bag. The Bottle Cage Double Adaptor is a side-by-side bottle mount that keeps your bottles within reach but out of the way of the rest of your gear.
The Bottle Cage Double Adapter mounts onto the bottle bosses of your down tube, with two bottle cages angled at 45-degrees or 90-degrees depending on the diameter of your frame tube. By angling the cages, you can now carry two water bottles outside the triangle, leaving room for your frame bag to fill the space.
The adapter is made from corrosion resistant stainless steel and weighs 55 grams. All the mounting hardware required is included in the package and it takes just a few minutes to install.
Cyclewerks recommends that when installing these adaptors it’s worth having a think about physics, and the limitations of your frame when you are loading two bottles on one set of frame bolts. When using the double bottle cage adaptor, with two full bottles, you are doubling the load that your frame water bottle bosses may have been designed for. To balance the load on the frame bottle bosses, particularly on light carbon frames, they recommend you keep a balanced load in the bottles side to side.
The Bottle Cage Double Adapter retails for NZ$40 or roughly $25 and the company will ship internationally if you get in contact with them. I sure wish I had this for my current bikepacking trip through Georgia as my full frame bag doesn’t allow for bottle cages.