Dual water bottle brackets on a single frame mount?
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.