The old Square D tandem breakers were front-to-back. The tandems since the 80's have been side-by-side. The new design gives the 2 pole devices the appearance of a quad breaker, but it is really (1) 2pole + (2) 1poles.
All UL Listed panels built since the 60's are CTL. So non-CTL breakers should only be installed in panels that are over 40 years old that have wiring diagrams showing which spaces are designed for tandem circuits.
My personal belief is that non-CTL tandem breakers should go the way of the old delta breakers.
