multiple heads as multiple screens?

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

I've got a laptop running 5.2 amd64.  In addition to its built-in
flatscreen, the display hardware has two other outputs on the back of
the machine, a DE15 ("VGA") and an DVI.  xrandr says there are outputs
VGA1, DVI1, and LVDS, and I can use options like --left-of to rearrange
them (though I had to hack on
xsrc/external/mit/xorg-server/dist/hw/xfree86/modes/xf86Crtc.c to get
the maximum virtual display size large enough to be useful).  But what
I would really like is for them to be separate X screens - the converse
of Xinerama, kind of.

I've been unable to make this happen.  Have I just missed something, or
is 5.2's Xorg really too stupid to do that?

If I run the server with +xinerama, it dies "Fatal server error:" /
"Caught signal 11.  Server aborting".  (I haven't yet gone chasing that
bug.)  -xinerama gives me the same behaviour as the default (which is
weird, because -xinerama is documented as disabling Xinerama, but the
resulting server claims to have it, listing "XINERAMA" among its
extensions, and it's doing Xinerama-like merging of multiple heads into
a single X display).

Seems like a weird sort of regression when the MIT server from way back
when could do it on substantially less powerful hardware.  Or is
+xinerama the right thing to do and I just need to figure out why it's
segfaulting?

I can provide Xorg.0.log if that would help; I didn't see anything
there that looked helpful to me, but that doesn't necessarily mean all
that much.

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