I know some Square D breakers will accept 2 conductors, but not all, correct? On the picture, I’m sure this is not acceptable, but would like another opinion, please!
The Square D QO tandems are only rated for single conductor terminations.
Those breakers are not rated for two conductors.
You will know when a breaker is listed for two conductors. It will say it on the breaker, and it will be obvious that it was designed for two conductors.
Personally I don’t like to apply the term double tap to breakers that allow two conductors. Double tapped is a defect.