Thanks again for your reply.
I don't understand why you would suggest it is a limitation of the window manager. If TwinView is indicating that Xinerama is not active, then why would a program continue to ask for Xinerama information?
That is, it seems quite reasonable that a program would be written as follows:
// Xinerama is enabled, so get information
info = XineramaQueryScreens(display, &num);
// Xinerama is not enabled, so can't get information
But if TwinView is returning false for XineramaIsActive, then such a program will come to the wrong conclusion. Do you really view this as a bug in the program, and not in the drivers?