My prime desktop XP computer (this one) has a pair of 5:4 ratio screens.
To the best of my experience it's the operating system (Windows) that controls the screen and not the program.
From memory when setting-up my two screens (on empty desktop space, right-click and select 'Properties', followed by the 'Settings' tab), it was possible to have two separate screens or one 'screen' that covered both.
It was sometime ago and I now regularly drag windows between screens, when the window is in variable size mode and not full screen mode. [In full screen mode a window fills just one screen.] [Right-click on Time (lower right) and 'Tile Windows ...' works on both screens, but separately.]
I have a photographic database program that can be sized to cover both screens. Then using the program's display options, arrange the size and position of the various parts to where I want them. The photo is then almost full-screen and the database info is shown on the other screen. But RB's options are somewhat limited.
I often think of how I managed on just one screen!
I've just bought a new laptop with W7 Pro - 64 bit and it has two external screen connectors. Documentation suggests that both can be supported simultaneously - so 3 screens!
Since getting ANRB, I've run it on an older XP desktop, but have returned today to the forum as I'm considering installing it on another computer that will have the existing screen of that older computer as a second screen.