This is important to me too.
How I currently use XnView (MP shown):
It would be great to have exactly this identical functionality in one (1!) XnView container window plus a customizable toolbar on the splitter between the two panels
housing various button controls such as copy, move, compare panels, synchronize locations, synchronize layouts, etc, etc.
The amount of time such a configuration would save me is huge.
When we are just looking at images, or working on a few of them, one browser is enough. But when we start managing 1000's of images in 100's of folders... well, one browser view at a time is an impediment.
The problem with my approach is that the two XnView instances know nothing about each other, so they cannot interact productively!
I was talking about a similar topic with someone the other day (professional digital artist) who was complaining that the Adobe Illustrator interface had oodles of functionality, but was so labor intensive to use (due to poor GUI design) that it was costing him money every time he moved the mouse to that toolbar way over there on the left of his 30" wide-screen monitor.
It's not that XnView is poorly designed (far from it!), but that it (and EVERY OTHER image browser) is less than optimally designed for some
of the tasks some
of us need it for. Take my word for it, using TC, or FreeCommander, or Dopus, or any other of the dual-pane file commanders (even with thumbs support and folder tabs etc etc etc) to manage images frankly stinks
compared to using XnView. Even the crude "two instance" manner that I employ is better than using TC to manage images, even considering all of TC's incredible dual-panel utility
The thing about dual browsers in XnView is that they could be a "discoverable" layout in the layout list that would be off
As a programmer I know it is mostly a matter of frames, linked lists, and pointers into the heap. It would not be like doubling XnView's file size or complexity or anything like that. Well, obviously memory footprint would increase.
Can't help it, I'll always hope for this. It would blow many user's minds (in a good way) to have dual pane functionality in an image browser.
I'd write my own, but I'm too old now.