Something

Section "InputDevice"
        Identifier      "Configured Mouse"
        Driver          "synaptics"
        Option          "CorePointer"
        Option          "Device"                "/dev/input/mice"
        Option          "Protocol"              "ExplorerPS/2"
        Option          "ZAxisMapping"          "4 5"
        Option          "Emulate3Buttons"       "true"
        Option          "VertScrollDelta"       "0"
        Option          "HorizScrollDelta"      "0"
EndSection

Section "InputDevice"
        Identifier      "Configured Mouse1"
        Driver          "auto"
        Option          "CorePointer"
        Option          "Device"                "/dev/input/mouse0"
        Option          "Protocol"              "IMPS/2"
        Option          "ZAxisMapping"          "4 5"
        Option          "Emulate3Buttons"       "false"
EndSection

Section "ServerLayout"
        Identifier      "Default Layout"
        Screen          "Default Screen"
        InputDevice     "Generic Keyboard"
        InputDevice     "Configured Mouse" "Corepointer"
        InputDevice     "Configured Mouse1"
EndSection


From Xorg.0.log:

(**) Configured Mouse1: Device: "/dev/input/mouse0"
(**) Configured Mouse1: Protocol: "ExplorerPS/2"
(**) Option "Device" "/dev/input/mouse0"
(**) Option "Emulate3Buttons" "false"
(**) Option "ZAxisMapping" "4 5"
(**) Configured Mouse1: ZAxisMapping: buttons 4 and 5
(**) Configured Mouse1: Buttons: 9
(II) XINPUT: Adding extended input device "Configured Mouse1" (type: MOUSE)
(II) XINPUT: Adding extended input device "Configured Mouse" (type: MOUSE)
(II) XINPUT: Adding extended input device "Generic Keyboard" (type: KEYBOARD)