I'm looking for a way to mount two water bottle cages on the seat tube. Is there any way to make this hold two water bottle cages on one bracket / mount point?
There are lots of clamp-on bottle cages (google link), letting you add bottles wherever you have space on your bike (this is just one design, and not a particularly good one IMO, better to have two clamps):
Or some clamps that let you mount conventional cages:
I suggest grabbing a couple of bottles and trying to fit them into the frame.
Also, Advantages of mounting bottle under frame has a discussion of places to get more bottles onto your frame that might give you some ideas. There are lots of clamp-on bottle cages available, so you really can just screw one on wherever there is space.
Finding a single part that will bolt onto your existing mounts is going to be hard. Putting the bottles side by side will make them too wide for you to ride comfortably, so I'm guessing you were imagining something like this?
That's a long, skinny bit of metal, that has to hold a litre or more of water... weighing a kilogramme or more. It's going to be hard to build.
Why two bottles? You'll have problems getting both in there unless your seat tube is really long.
Why not one larger bottle?
Or a longer bottle which won't be as wide
Between your downtube and seattube holders you will have 1.5-1.6 Litres of liquid, and more if you use bigger bottles. If ~3 litres is not enough, go for a bigger hydration pack.
Other options are
- hang one or two off the rear of the seat, (high and aft weight)
- One behind the seatpost (high and aft)
- one or two on the handlebars (high and forward weight, plus gets in the way of cockpit, not aero, and weights the steering like a basket)
- under the downtube ( ++Low and central, but very hard to get to while riding
- On the fork legs ( bad aero, weights the steering, risk of touching spinning wheel)
- Frozen one in the middle back pocket - works okay but is still high/aft weight, and its on the rider too.
I fell over a solution you might be interested in:
the WOLF TOOTH B-RAD 4
It's exactly like @Moz drawing, just irl. See https://bikepacking.com/gear/wolf-tooth-b-rad/ (quite far down) for details
Please notice: i neither own nor know that product in any more detail