I'm currently using multiseating within X and using:
AMD Athlon 1.6Ghz, 512MB Ram, 
ATI Rage128 (AGP), (FIRST SEAT)
Matrox Millenium II (PCI), (SECOND SEAT)
Gentoo 2006.1,
Gdm 2.6.14

If the second seated device doesn't have an active keyboard/mouse/monitor it can use then it will crash the entire system.

If sharevts is used on the Standard (First seat) server, then tty artifacts leak into it's X Session, ie I can type into the first tty within X.

From gdm custom.conf
0=Standard
1=Extra

[server-Standard]
name=Standard server

command=/usr/X11R6/bin/X0 :0 -deferglyphs 16  -novtswitch -isolateDevice PCI:1:0:0 -layout seat0
flexible=false

[server-Extra]
name=Extra Server
command=/usr/X11R6/bin/X1 :1 -sharevts -deferglyphs 16 -isolateDevice PCI:0:9:0 -novtswitch -isolateDevice PCI:0:9:0  -layout seat1
flexible=false

My xorg.conf

Section "ServerLayout"
	
	Identifier     "main"
	Screen 0 	"Screen0" 0 0
	Screen 1	"Screen1" 0 0
	InputDevice	"Mouse0" "CorePointer"
	InputDevice	"Keyboard0" "CoreKeyboard"
	InputDevice    "NoMouse" "CorePointer"
	InputDevice    "NoKeyboard" "CoreKeyboard"
EndSection

Section "Serverflags"
	Option "DefaultServerLayout" "main"
#	Option "DontVTSwitch" 	"yes"
#	Option "DontZap" 	"yes"
EndSection

Section "Files"
	RgbPath      "/usr/share/X11/rgb"
	ModulePath   "/usr/lib/xorg/modules"
	FontPath     "/usr/share/fonts/misc/"
	FontPath     "/usr/share/fonts/TTF/"
	FontPath     "/usr/share/fonts/OTF"
	FontPath     "/usr/share/fonts/Type1/"
	FontPath     "/usr/share/fonts/CID/"
	FontPath     "/usr/share/fonts/100dpi/"
	FontPath     "/usr/share/fonts/75dpi/"
EndSection

Section "Module"
	Load  "extmod"
	Load  "dbe"
	Load  "record"
	Load  "xtrap"
	Load  "dri"
	Load  "glx"
	Load  "freetype"
	Load  "type1"
	Load  "GLcore"
	Load  "fbdevhw"
EndSection

### seat 0 ###

Section "ServerLayout"
	Identifier	"seat0"
	Screen	0	"Screen0" 0 0
	InputDevice	"Mouse0" "CorePointer"
	InputDevice	"Keyboard0" "CoreKeyboard"
	#InputDevice	"NoKeyboard" 
	#InputDevice	"NoMouse"
	
EndSection

Section "InputDevice"
	Identifier  "Keyboard0"
	Driver      "kbd"
EndSection

Section "InputDevice"
	Identifier  "Mouse0"
	Driver      "mouse"
	Option	    "Protocol" "auto"
	Option	    "Device" "/dev/psaux"
	Option	    "ZAxisMapping" "4 5 6 7"
EndSection

Section "Monitor"
	Identifier   "Monitor0"
	VendorName   "Monitor Vendor"
	ModelName    "Monitor Model"
EndSection

Section "Device"
	Identifier  "Rage 128"
	Driver      "r128"
	VendorName  "ATI Technologies Inc"
	BoardName   "Rage 128 Pro Ultra TF"
	BusID       "PCI:1:0:0"
EndSection

#Section "Device"
#	Identifier "Mach64"
#	Driver	   "mach64"
#	BusID	   "PCI:00:0a.0"
#EndSection

Section "Screen"
	Identifier "Screen0"
	Device     "Rage 128"
	Monitor    "Monitor0"
	DefaultDepth 24
	SubSection "Display"
		Viewport   0 0
		Depth     1
	EndSubSection
	SubSection "Display"
		Viewport   0 0
		Depth     4
	EndSubSection
	SubSection "Display"
		Viewport   0 0
		Depth     8
	EndSubSection
	SubSection "Display"
		Viewport   0 0
		Depth     15
	EndSubSection
	SubSection "Display"
		Viewport   0 0
		Depth     16
	EndSubSection
	SubSection "Display"
		Viewport   0 0
		Depth     24
	EndSubSection
EndSection

Section "DRI"
    Mode 0666
EndSection

### Seat 1 ###

Section "ServerLayout"
	Identifier 	"seat1"
	Screen 0	"Screen1"
	InputDevice	"NoKeyboard" 
	InputDevice	"NoMouse"
EndSection

Section "Device"
	Identifier	"Matrox"
	Driver		"mga"
	BusID		"PCI:0:9:0"
EndSection

Section "Monitor"
	Identifier 	"Monitor1"
	VendorName 	"vend"
	ModelName	"model"
EndSection

Section "Screen"
	Identifier	"Screen1"
	Device		"Matrox"
	Monitor		"Monitor 1"
	DefaultDepth	16
	SubSection	"Display"
		ViewPort 0 0
		Depth 24
	EndSubSection
EndSection

### Void Keyboards/Mice ###

Section "InputDevice"
	Identifier	"NoKeyboard"
	Option		"CoreKeyboard"
	Driver		"void"
EndSection

Section "InputDevice"
	Identifier	"NoMouse"
	Option		"CorePointer"
	Driver		"void"
EndSection



I have used void devices for when I'm working without an extra mouse/keyboard and it doesn't crash the machine. But no monitor will still crash the machine.