I've been after this too.
Code:
pinebook 18:26:48 ~> xinput list
⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ <default pointer> id=6 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ <default keyboard> id=7 [slave keyboard (3)]
pinebook 18:29:53 ~> xinput list-props 6
Device '<default pointer>':
Device Enabled (115): 1
Coordinate Transformation Matrix (116): 1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000
Device Accel Profile (229): 0
Device Accel Constant Deceleration (230): 1.000000
Device Accel Adaptive Deceleration (231): 1.000000
Device Accel Velocity Scaling (232): 10.000000
Device Node (233): "/dev/wsmouse"
Button Labels (234): "Button Left" (118), "Button Middle" (119), "Button Right" (120), "Button Wheel Up" (121), "Button Wheel Down" (122), "Button Horiz Wheel Left" (123), "Button Horiz Wheel Right" (124), "Button Unknown" (228), "Button Unknown" (228)
Mouse Middle Button Emulation (235): 1
Mouse Middle Button Timeout (236): 50
This is on NetBSD, but the commands should apply to anything running Xorg.
In this case, the emulation is already enabled, but can be toggled with:
xinput set-prop 6 235 1
However, in my case, I'm not seeing the button being emulated.
Running xev -event button seems to indicate the trackpad isn't reporting 2 buttons pressed simultaneously. If I have a button down event on button 1 (left), down on button 3 (right), I get no event reported for button 3. Similarly, the button up event is always for the first button down.