Some time ago i tried to have different thumbnails size on the two sides of a double lister window. I did'nt manage, and i'd been told here that it was not possible.
I found an happy bug which can allow that.
The rule is simple: a covered tab does'nt change in thumbnail size. So just cover the lister where you want to keep the thumbnail size, and change (or select) the size of the thumbnail on the other side.
It's so simple that i suppose that many guys have found it. But maybe not all guys ?
Nice find. 
Now the question is, is it a bug of a feature?
You can set the thumbnail size independently in different listers as well (I think you knew that, but maybe other people don't). Looks like Opus is happy enough to have two lists in the same dual-lister using different sizes too, it's just that the size slider applies to both of them when you move it.
Perhaps all it needs is for the slider to get an argument specifying whether it affects both sides, or just the source/dest/left/right of a dual lister?
Well this also 'sort' of circles back to the idea of having toolbars associated/docked to particular sides of dual-pane listers eh? Where the buttons on a lister pane-bound toolbar would inherently operate against files selected in that side of the lister?
But of course 'both/source/dest/left/right' args keep things in line with other functions in Opus.