I was thinking IF one had only the option to stack ICs, that encasing the driver in Arctic Alumina could help counter the heat concentration. Why do you suggest against stacking ICs otherwise?
Encasing anything in Arctic Alumina doesn't help heat dissipation. That's not how Arctic Alumina (or Arctic Silver, etc) works. It's not a magic heat-seeking vampire that turns Florida into northern Canada.
Thermal epoxies/grease/paste/etc work by filling all the little tiny gaps between two heat conducting surfaces that you want to attach. In effect, it creates a greater overlap of the two surfaces than you would otherwise have if they were just in contact, but without the substance in between. However, thermal goops are not good heat conductors when used to fill large gaps.
"Encasing the driver in Arctic Alumina" is likely to be
a lot worse, and cause a lot more heat buildup, than simply leaving it open to the air. It would all depend on your host, but this strikes me as a loosing strategy 99% of the time.
In terms of stacking the ICs (as opposed to the drivers), my concern is with heat. You're now dissipating most of the top IC's heat through the bottom IC, meaning that the bottom IC has to do double-duty. Not only that, but it is doing double duty, without the benefit of it's own top-surface to dissipate heat through (either to the air, to the Aixiz module, or to the underside of another PCB, as the case may be). If the ICs get hot enough, they un-solder themselves.
More to the point, even if you're only
partially concerned about the above, why take that approach when it confers absolutely no benefit? I guess you save 1.6mm of thickness? (the thickness of a PCB) That's a really silly reason to adopt a sloppy approach to building.