There's a mistake in your analysis: You want to neglect the 24V transformer by generating the 24V from 12V supply. But if you do this, the rating of the transformer of 12V supply must tremendously be increased! If you want 24V at 1A, for instance, your transformer must additionally deliver more than 2A!! So, you will not waste lots of space if you choose two tiny transformers instead of a big one.

In situations, where only minimum heat dissipation is allowed, I use step down switchers, like LM2674 or LM2675. I make one high supply voltage by the help of mains transformer and step down all the needed voltages from this supply. I do this with SMD parts and use super low ESR-caps from panasonic to keep the ripple little. I do also use an output ripple filter and a solid ground plane with lots of vias. The result is as good as buying a much more expensive switcher from traco or similar. Costs play a role if you need several supply voltages...

Kai