I got my second monitor up and running on FreeBSD this afternoon (running off an old 3dfx Voodoo 3 PCI). There are two ways you can run dual monitors with Xorg. With or with out Xinerama.
With Xinerama turned on, it handles the different monitors similar to Windows. It acts like one large monitor.
In Xorg with Xinerama disabled each monitor acts like is own instance. For example you cannot move windows between them, they feel pretty separate. What you do get is 8 Virtual desktops (4 for each screen). It is pretty cool. So you can change between applications and desktops. It is like running two computers with one mouse and keyboard. It is pretty interesting.
Although I am currently running with Xinerama as I am used to coming from a Dual windows setup.
My only main problem at the moment is that I cannot get 3D (via DRI) working on my Radeon. There is an issue with Nforce2 motherboards or something :(