I have one made by Minoura. It clamps to the horizontal section of the bars using one rubber sleeved U-bolt on either side of the stem. It has provisions for mounting two conventional water bottle cages to it via the normal two screws. You could use any water bottle cages you choose.

I went to the Minoura web site. They do not show a dual bottle unit but have several different styles of single bottle holders. Buy two!

My recollection is that when the two bottles are full (heavy), the entire affair tends to bounce around (bobble) a lot.

I also found this from Nashbar being sold on Amazon:

http://www.amazon.com/Nashbar-Handle...ef=pd_sim_sg_2