You would need normal power to power a zwave device. The power emitted by the garage door opener isnt enough to power a z wave device.
The GoController would definitely work but you have to have 2 if you have 2 garages.
GoControl Z-Wave Garage Door Opener Remote Controller
Also the My Q works (it’s a little harder to integrate into smartthings but the normal app works well). I believe 1 device can control 2 doors. The device links up to the garage door like a remote opener. All you have to do is plug it into a wall. Also your garage door opener needs a “sync” button (I believe the opener should be good if it was made after 1993)
I think both devices we would serve your purpose but neither would replace those buttons. That is if by “on/off” you mean “open/close”.