Alisdair,
...Indeed (good researching).
However, maybe the following is of some help 9?)
I think the word coniuncta refers to being joined in the sense of "The Newlyweds."
Spuler, 1908. Die Schmetterlinge von Europas, Stuttgart, 1901-1910 (316R)
Quite how that is particular to this species, however, I don't know.
In the close relative Catocala sponsa, 'sponsa' means 'bride' - so there may be some folk lore in play, methinks.
Whatever the reasoning, I guess it may be the same as that behind Spuler naming Zygaena coniuncta.
See also here re mis-spelling of 'coniuncta' as 'conjuncta'
http://en.wikipedia.org/wiki/Catocala_coniunctaWhilst on the subject...
'Catocala' derives from the Greek kato (below) and kalos (beautiful), referring to the nice red underwings.
That's a lot easier to understand.
Mike