Well, how badly do you want to spend zero money? If you are in to a little effort, install Apple's X11. Then go get DarwinPorts and pick from any of a large number of two pane FTP clients for Linux that run as part of DarwinPorts. Apple's X11, DarwinPorts and all of the software packages it supports are all absolutely free (released under GPL I believe). The open source world is literally crawling with two pane FTP clients.
I use the venerable old Midnight Commander myself, but gFTP is very well regarded as well, along with LOTs of others. The most capable of them all, IMHO, is Konqueror, a file manager and FTP client and ... a LOT of other stuff. In fact a great many open source dual pane file managers support FTP client capability in one or both of their panes.
Back in the "pay for it" world, have you considered CaptainFTP? A nice package for Mac that I got more or less free - it was free on a DVD that comes with my MacFormat magazine subscription. Of course, that subscription isn't free, so I paid for it in the end!