What I need is either a "double female" connector (which would be the
opposite of the "double male" ones, and definitely not the 'hoze repair'
item), or alternatively a threaded-tap-to-female connector.
There are thousands of threaded-tap-to-male connectors, and adding the
"double female" to that would create the threaded-tap-to-female
I can't find these anywhere; the best, but very messy, workaround being
a pair of back-to-back hose-to-female connectors (and a two-inch hose).
[Not worried if it's an official yellow-and-four-times-the-price one, or
a plug-compatible in green/blue/whatever.]