First, you simply plug in the second framebuffer; plug in the monitor, make sure you have the correct device drivers installed and do a reconfiguration boot.
The X server is started through the dt/config Xservers file; you'll need to modify it when you change your X server configuration:
# Never edit the /usr/dt/config files in place mkdir -m 755 -p /etc/dt/config cp /usr/dt/config/Xservers /etc/dt/config
Edit the Xservers file and change the line with "local_uid@console" to suit your needs; the Xsun(1) describes what arguments to pass.
Here are some examples (lines split for clarity):
# FFB as left monitor, PGX (m64) to the right (two ways) :0 Local local_uid@console root /usr/openwin/bin/Xsun :0 -nobanner \ -dev /dev/fbs/ffb0 -dev /dev/fbs/m640 :0 Local local_uid@console root /usr/openwin/bin/Xsun :0 -nobanner \ -dev /dev/fbs/m640 -dev /dev/fbs/ffb0 left# FFB, PGX (m64) underneath :0 Local local_uid@console root /usr/openwin/bin/Xsun :0 -nobanner \ -dev /dev/fbs/m640 -dev /dev/fbs/ffb0 top
# 2x FFB, using a default 24 bit visual instead of the standard 8 :0 Local local_uid@console root /usr/openwin/bin/Xsun :0 -nobanner \ -dev /dev/fbs/ffb0 defdepth 24 -dev /dev/fbs/ffb1 defdepth 24