my bike did not come with a 2nd set of brazeons for a 2nd cage so my DH mounted a cage holder to my handlebars. At first it did not seem stable, as the holder itself would rotate around the bar, so what he did was put a plastic shim underneath basically to absorb any extra space around the handlebar. It has stayed rock solid ever since. I seem to be one of the only ones with this kind of set up.

That said, I still don't drink on the bike well. It has to be 100% flat and very low traffic for me to even think about it. The other day I got the bottle out from the lower cage only to realize I was on a downhill so... bottle or me... I chucked the bottle so I could get both hands on the bars again. [then went and retrieved] I can't imagine how the riders who can eat, put their coats on or off, or perform TdF gear distribution do it. Maybe it will come with time, but I am skeptical.

I have however mastered swigging at stopsigns, which seems to work well enough for most days.