One thing window can't do is let you run two X sessions on different virtual
   terminals [...].

I personally do not see any need for this at all.  I'm currently running
ctwm and it allows for multiple workspaces.  The number of workspaces
(call them virtual X sessions) is user configurable and they can share
windows between different workspaces.  Granted, if you hose the X server,
all workspaces are blown away, but that is not a real big deal.