I'm curious about this, because this is how I typically do it, assuming the - wires connecting between the PCB's would hopefully be sufficient for thermal conductance. I was really thinking this isn't much worse than typical ways a driver is mounted, because they are usually just mounted on a thread of a ledge, and maybe 2-3 solder contact pts, if there a brass mount or copper mount available. If the pill is all aluminum, options are usually worse - I've found if you can press fit the driver in, it usually it is not electrically making contact, so even though it looks like the dirver is sitting on the shelf making contact form the copper ring on the driver to that small ledge, it's not really making contact at all, so you have to jam in some copper material to get a good electrical contact, but then, the thermal transfer is limited to just the contact surface doing the elctrical contact.
Would simply adding more wire or copper material connecting between the two driver grounds (battery neg.) be better? I know O-L seems to prefer something like that on his high amp 7135 based drivers. I believe he sees a lot of thermal sag with simple stacked 7135's, but if he spreads them out to another slave board, and adds copper heat sinking, it reduces the heat sag.
Or could I simply solder on copper material to the driver outer ring, adding mass, and adding more surface area to better dissapate the heat?