Chris Hogg wrote:

Connecting the two butts at the bottom is by far the better way to do
it IMO. You only need one tap, and you only need to drill one extra
hole, and that in the butt that is going to have the tap, as you use
the tap-hole in the other butt for the connector fitting. But you do
need both butts to be empty first, on their sides, and the assistance
of a small person to crawl inside and screw up the nut. When the nut
starts to grip, tightening it from the outside is usually practical,
so the small person doesn't have to do it from the inside.

Given that various surrey/essex/warwick/tech flanges exist which can be
installed into H/W cylinders from the outside, don't suitable flanges
exist for water butts?