You are using a nunchuk so you will need to use the canbus to control the second VESC. I have not tried splitting the leads on a wii dongle yet but it may work. Better to use the canbus though.
If you where using a conventional FHSS transmitter you could split the receiver output leads and control both VESC through the ppm input. Be sure to only have the red wire coming from one vesc if you do this.