Eu tenho a resposta na lista de discussão do xorg:
Using it this way is nearly impossible: for some historical reasons, inactive server stops drawing. Well, not just historical: it is a reasinable approach to stop wasting resources to draw unseen things.
However, you can start virtual X server (like standard Xvfb),and draw on it just fine.
You can even try to work interactively with programs on that server via something like x2x or x11vnc or x2godesktopsharing, however, usually interactive experience is worse than with console server.
Also, no hardware 3D acceleration will be available. It also works only on active console currently. If you need it, you can try to use some server-in-window servers like xpra, however, it is not perfect (and all attempts to implement it in the past -- Xgl, AIGLX was not perfect also).