Logging into stretch with crosvm gpu seems to result in one sommelier instance starting correctly and the other not.  

Jan 18 02:06:15 stretch-x systemd[1]: Started Update UTMP about System Runlevel Changes.
Jan 18 02:06:15 stretch-x sommelier[110]: Mesa 18.2.0-devel implementation error: Invalid GLSL version in shading_language_version()
Jan 18 02:06:15 stretch-x sommelier[110]: Please report at https://bugs.freedesktop.org/enter_bug.cgi?product=Mesa
Jan 18 02:06:15 stretch-x sommelier[110]: Failed to initialize glamor
Jan 18 02:06:15 stretch-x sommelier[110]: Failed to initialize glamor, falling back to sw
Jan 18 02:06:15 stretch-x sommelier[110]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/cyrillic
Jan 18 02:06:15 stretch-x sommelier[110]: _FontTransOpen: Unable to find transport for
Jan 18 02:06:15 stretch-x sommelier[110]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/Type1
Jan 18 02:06:15 stretch-x sommelier[110]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/100dpi
Jan 18 02:06:15 stretch-x sommelier[110]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/75dpi
Jan 18 02:06:16 stretch-x sommelier[109]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/cyrillic
Jan 18 02:06:16 stretch-x sommelier[109]: _FontTransOpen: Unable to find transport for
Jan 18 02:06:16 stretch-x sommelier[109]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/Type1
Jan 18 02:06:16 stretch-x sommelier[109]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/100dpi
Jan 18 02:06:16 stretch-x sommelier[109]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/75dpi
Jan 18 02:06:16 stretch-x systemd[91]: Started X11 sommelier at display 1.
Jan 18 02:06:16 stretch-x systemd[91]: Started X11 sommelier at display 0.
Jan 18 02:06:16 stretch-x systemd[91]: Started Chromium OS Garcon Bridge.
Jan 18 02:06:16 stretch-x systemd[91]: Reached target Default.
Jan 18 02:06:16 stretch-x systemd[91]: Startup finished in 209ms.
Jan 18 02:06:16 stretch-x systemd[1]: Started User Manager for UID 1000.
Jan 18 02:06:16 stretch-x garcon[209]: I0118 02:06:16.196757805     211 server_builder.cc:247]      Synchronous server. Num CQs: 1, Min pollers: 1, Max Pollers: 2147483647, CQ timeout (msec): 1000

davidriley@stretch-x:~$  DISPLAY=:0 glxgears & pid=$!; echo got $pid; sleep 1; grep dri.so /proc/$pid/maps; fg
[1] 372
got 372
libGL error: MESA-LOADER: failed to retrieve device information
MESA-LOADER: failed to retrieve device information
MESA-LOADER: failed to retrieve device information
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
78c9b1501000-78c9b1e3d000 r-xp 00000000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
78c9b1e3d000-78c9b203c000 ---p 0093c000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
78c9b203c000-78c9b2077000 r--p 0093b000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
78c9b2077000-78c9b2083000 rw-p 00976000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
DISPLAY=:0 glxgears
^C
davidriley@stretch-x:~$ DISPLAY=:1 glxgears & pid=$!; echo got $pid; sleep 1; grep dri.so /proc/$pid/maps; fg
[1] 375
got 375
78aaeae5d000-78aaeb799000 r-xp 00000000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so
78aaeb799000-78aaeb998000 ---p 0093c000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so
78aaeb998000-78aaeb9d3000 r--p 0093b000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so
78aaeb9d3000-78aaeb9df000 rw-p 00976000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/swrast_dri.so
DISPLAY=:1 glxgears
^C

Restarting the sommelier instance fixes the issue:
davidriley@stretch-x:~$ systemctl --user restart sommelier-x@1
davidriley@stretch-x:~$ DISPLAY=:1 glxgears & pid=$!; echo got $pid; sleep 1; grep dri.so /proc/$pid/maps; fg
[1] 437
got 437
libGL error: MESA-LOADER: failed to retrieve device information
MESA-LOADER: failed to retrieve device information
MESA-LOADER: failed to retrieve device information
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
79baab4b3000-79baabdef000 r-xp 00000000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
79baabdef000-79baabfee000 ---p 0093c000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
79baabfee000-79baac029000 r--p 0093b000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
79baac029000-79baac035000 rw-p 00976000 00:24 4678                       /usr/lib/x86_64-linux-gnu/dri/virtio_gpu_dri.so
DISPLAY=:1 glxgears

Jan 18 02:09:45 stretch-x systemd[91]: Stopping X11 sommelier at display 1...
Jan 18 02:09:45 stretch-x systemd[91]: Stopped X11 sommelier at display 1.
Jan 18 02:09:45 stretch-x systemd[91]: Starting X11 sommelier at display 1...
Jan 18 02:09:45 stretch-x sommelier[391]: gbm: failed to open any driver (search paths /usr/lib64/dri)
Jan 18 02:09:45 stretch-x sommelier[391]: gbm: Last dlopen error: /usr/lib64/dri/virtio_gpu_dri.so: cannot open shared object file: No such file or directory
Jan 18 02:09:45 stretch-x sommelier[391]: failed to load driver: virtio_gpu
Jan 18 02:09:45 stretch-x sommelier[391]: gbm: failed to open any driver (search paths /usr/lib64/dri)
Jan 18 02:09:45 stretch-x sommelier[391]: gbm: Last dlopen error: /usr/lib64/dri/kms_swrast_dri.so: cannot open shared object file: No such file or directory
Jan 18 02:09:45 stretch-x sommelier[391]: failed to load driver: kms_swrast
Jan 18 02:09:45 stretch-x sommelier[391]: warning: unknown scale factor: 444
Jan 18 02:09:45 stretch-x sommelier[391]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/cyrillic
Jan 18 02:09:45 stretch-x sommelier[391]: _FontTransOpen: Unable to find transport for
Jan 18 02:09:45 stretch-x sommelier[391]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/Type1
Jan 18 02:09:45 stretch-x sommelier[391]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/100dpi
Jan 18 02:09:45 stretch-x sommelier[391]: _FontTransOpen: Unable to Parse address  /usr/share/fonts/X11/75dpi
Jan 18 02:09:45 stretch-x systemd[91]: Started X11 sommelier at display 1.
Jan 18 02:09:47 stretch-x sommelier[391]: resource (nil) for handle 4, pfd=12
Jan 18 02:09:47 stretch-x sommelier[391]: resource (nil) for handle 3, pfd=12
Jan 18 02:09:47 stretch-x sommelier[391]: resource (nil) for handle 7, pfd=12
Jan 18 02:09:53 stretch-x sudo[440]: davidriley : TTY=console ; PWD=/home/davidriley ; USER=root ; COMMAND=/bin/journalctl --no-pager