multi-monitor support is still very experimental, and not 
very easy to setup. Here's a copy of a message I sent to 
the mailing list:


First: only nvidia, 3dfx, sis and i740 cards can be secondary cards,
(though the primary can be anything, including vesa).
Second: as far as I can understand, once both cards are enabled, there
ought to be a resource contention (in hardware) between the cards for
the vga regions (IO 0x3c0-0x3df and MEM 0xa0000-0xbffff). I don't know
how it is supposed to be solved, but in practice, if I ignore it in my
system, it just works.

If anybody else has successfully run svgalib on a secondary card, please
tell me, and if you had to do something different from what I did, a
detailed report to the list might help others.

Here's my setup:

I have two vga cards: AGP (1:0.0) nVidia TNT2 and (PCI0:0f.0) Riva128.
The AGP card is the primary (in the BIOS option "initialize AGP card
first" is selected). My boot sequence includes the following code to
initialize the secondary card:

/usr/local/sbin/vbios.x86emu
/usr/local/sbin/vga_reset
/sbin/setpci -s 00:0f.0 COMMAND=2

the vbios.x86emu is the same as distributed in x86emu-0.6 (from ftp.scitechsoft.com) 
except that in debug.h the line
#define CONFIG_ACTIVE_DEVICE 1
is replaced by
#define CONFIG_ACTIVE_DEVICE 0
(so that the active device is not initialized by x86emu).

the setpci line enables memory access (of the drivers supporting
secondary card, nv3 and i740 use only memory mapped IO, sis and banshee
need I/O access as well, so you need COMMAND=3). The COMMAND register
might have other bits set, so you might try to see its value first, and
then set the 0,1 bits.

I have two config files in /etc/vga:
libvga.config.0 - for the scondary card:

novccontrol
secondary
include /etc/vga/libvga.config.common
HorizSync 30 54
VertRefresh 40 120

The novccontrol and secondary options are necessary for a secondary
card. The include statement is for a card with common configurations -
mouse, keyboard, etc.

libvga.config.1 - for the primary:

PCIStart 1 0
include /etc/vga/libvga.config.common
HorizSync 30 115
VertRefresh 40 160

The pcistart option is necessary to make sure that svgalib skips the
first card (in PCI order), and so finds the AGP card.

to run a program on the primary card, in the usual way, I do

export SVGALIB_CARD=1
/svgalib/demos/vgatest

to run a program on the secondary card:

export SVGALIB_CARD=0
/svgalib/demos/vgatest

There are a few differences for a program running on secondary card:

- The program is not aware of VC switches, so you can switch to another
  console, and the program will still run (and display).
- The program can't event system (mouse or raw keyboard).
- Standard VGA modes and paged memory modes might have problems.

What this limitations mean is that usual svgalib program might not be
useful for a secondary card. I use the secondary display for
watching TV, movies and slideshows. Eye candy programs (such as
synaesthesia) might also work, after a few changes, but interactive
programs (zgv, games) are unusable right now. I intend to add support
for Vojtech Pavlik's input event system to svgalib-2.0, so that
interactive program will be usable as well.