I wrote about this dual water bottle cage last year.
I finally got around to taking pictures of the mount for the rings. It’s under my Brompton’s saddle, and stays permanently on the bike.
The support isn’t in the way when the cages are detached, and I don’t notice it when my bicycle is in everyday use.
When attached, the cages add minimally to Basil’s length, and slightly to his weight; those premiums are not too much to pay for the convenience of having three full water bottles handy on a hot day.
This is also a convenient way to go at an event where hydration bladders are banned, as they are at the 5 Boro. The 5 Boro administration recommends carrying three water bottles, but does not allow packs or panniers; Basil also has a trim water bottle holder on his handlebars, which, combined with these cages, allows me to carry all three bottles directly on my Brompton.