No, xbindkeys can't detect simultaneous button presses. But you don't need that to emulate a middle click on left+right click: it's a built-in feature.
You can turn it on with the xinput program. See Make mouse movements scroll when the middle button is held downMake mouse movements scroll when the middle button is held down for a tutorial. In a nutshell:
- Run
xinput listto list available input devices. Locate the name of your mouse device; I'll useMy Mouse Devicein my answer. - Run
xinput list-props 'My Mouse Device'to list the available properties. One of them should beEvdev Middle Button Emulation. - Run
xinput set-prop 'My Mouse Device' 'Evdev Middle Button Emulation' 1
Once you have the command right, add it to your autostart list, i.e. create a .desktop file in ~/.config/autostart. See Running program on startup (Fedora 22/ 2015)Running program on startup (Fedora 22/ 2015) for an example.