Hi Andy,
I really appreciate your help!
I believe you're correct in that I wouldn't be able to start the X server and have it only produce output on one of my monitors, as they are necessarily part of a single "display" (in OS X, at least). Barring that, however, I wanted to be able to at least keep X11 from creating a single screen that spanned both my monitors (the problem with this is that it doesn't recognize the separate monitor geometries, so large chunks of the virtual spanning screen were inaccessible, and it also made dealing with X apps that are none dual-head friendly a pain). On my linux machine, I was able to get this behavior by default (i.e., separate screens named '😮.0 and 😮.1' under a single X display), and could get monitor spanning only after enabling the Xinerama extension.
I fiddled with Apple's X11, which I'd installed on my Desktop, to no avail. Nowhere could I disable the screen-spanning behavior. On a hunch, I decided to try out XDarwin, however, and lo and behold --- a checkbox for enabling/disabling Xinerama! In addition, XDarwin seems to be better behaved than Apple's X11 when it comes to correctly sourcing some of my $HOME/.X* configuration files (e.g., I always had to run xmodmap ~/.Xmodmap manually under X11, even after putting the command in my .xinitrc).
Anyway, that basically gave me what I needed, so I'm a happy camper. Just thought I'd post this in the unlikely event that someone else is in the same situation. Oh, and if you're curious, the reason I needed to do this in the first place was because I wanted to run the ratpoison windows manager fullscreen --- great for dealing with ADD!
