To use an independent mouse for each of your independent X servers/sessions, you just have to modify the input section of the XFree configuration files to point to the proper device files.

Use /dev/input/mouse[n], where n is the number of your mouse starting from 0:

You shouldn't use /dev/input/mice because it merges the input from all mouse devices.

Here is my configuration before modifications:

------------------------------------------------------------------------------------------------

# **********************************************************************
# Pointer section
# **********************************************************************

Section "InputDevice"

    Identifier  "Mouse1"
    Driver      "mouse"
    Option "Protocol"    "IMPS/2"
    Option "Device"      "/dev/psaux"
    Option "ZAxisMapping" "4 5"

# ChordMiddle is an option for some 3-button Logitech mice
    Option "Emulate3Buttons"
#    Option "ChordMiddle"

EndSection

------------------------------------------------------------------------------------------------
      

After modifications for the first X server:

------------------------------------------------------------------------------------------------

# **********************************************************************
# Pointer section
# **********************************************************************

Section "InputDevice"

    Identifier  "Mouse1"
    Driver      "mouse"
    Option "Protocol"    "IMPS/2"
    Option "Device"      "/dev/input/mouse0"
    Option "ZAxisMapping" "4 5"

# ChordMiddle is an option for some 3-button Logitech mice
    Option "Emulate3Buttons"
#    Option "ChordMiddle"

EndSection

-----------------------------------------------------------------------------------------------
      

For the second X server:

-----------------------------------------------------------------------------------------------  
# **********************************************************************
# Pointer section
# **********************************************************************

Section "InputDevice"

    Identifier  "Mouse1"
    Driver      "mouse"
    Option "Protocol"    "IMPS/2"
    Option "Device"      "/dev/input/mouse1"
    Option "ZAxisMapping" "4 5"

# ChordMiddle is an option for some 3-button Logitech mice
    Option "Emulate3Buttons"
#    Option "ChordMiddle"

EndSection
-----------------------------------------------------------------------------------------------
      

and so on ...