Two pane file manager for Mountain Lion?
Forklift is nice, has support for FTP + SFTP + Amazon S3 + WEBDav. A bit pricey, but I got it on sale and there were a few discounts again after that so maybe worth a wait.

Screenshot of Forklift http://shrani.si/f/F/wu/3VAuwxPt/screen-shot-2013-06-27-a.png

You have mentioned Krusader (and wish to use open source software), it can be run on Mac via MacPorts. After installing MacPorts use:

sudo port install krusader


I doubt it will look good, but It should work. It supported Aqua look years ago, so no X11 was needed, but I have no idea how it stands now.