FWIW I have a couple of Square D 'double' 15 amp breakers. These are
doubles (Not double pole).
By that I mean these are two 15 amp single pole breakers, both on the same
'leg' (or incorrectly called 'phase' side!);
BUT they fit in the same SINGLE SAME PHYSICAL SPACE as a normal 'single' 15
amp single pole breaker.

This allows the use of one of these 'doubles' to 'crowd up' say a couple of
#14AWG 15 amp lighting circuits. That saves one space which then allows the
placing of a proper double pole breaker, of the appropriate rating, which in
adjacent spaces spans both of A and B legs to give 230 volts.

I can look at the Square D type number if you wish.

Interesting (sensible) post and answers IMHO.