I love that you can change the setting and allow multiple instances -- and then (in the Browser) you can drag & drop to copy/move images between them. That makes the need for dual panes much less urgent in my opinion.
To make things quicker/easier, how about this: If you right-click a folder within the program, offer something like "Open in new instance"? (That option could be grayed out or hidden if the multiple-instances settings prevents multiple instances.)
Open folder in new instance
Moderators: XnTriq, helmut, xnview