If you are looking for usage from just i3, I am using xrandr to enable/disable multi-monitor:

    $mod+Shift+p exec "xrandr --output HDMI1 --auto --right-of eDP1"

(that with enable second screen through HDMI when Win+Shift+p is pressed)

    $mod+Shift+o exec "xrandr --output HDMI1 --off"

(that disables second screen)

    bindsym $mod+Shift+x move workspace to output HDMI1

(that moves an entire workspace to the 2nd screen or any screen with the given id)