When I'm in Flatview mode and open an image with the command Show LISTSIBLINGS the generated next/prev list doesn't work correctly. Only files from one of the merged folders will be selected by the Next/Previous buttons.
Try AUTOFILELIST instead of LISTSIBLINGS.
It's currently a private/hidden/undocumented argument, but I think it does what you want, and we can make it officially supported.
Thanks, AUTOFILELIST is working fine with Flatview.
Making a note for AUTOFILELIST in my little black notebook. o)
What other undocumented features are out there.. ? o)