Kohoa -
You don't really "need" a protocol to communicate between the two boards... the CAN is implemented via the hardware - all you would have to do is have a message ID, 0x120 for example, and then your body of information, you should be able to get that to go across.
Bosch is the orginator of the CAN protocol if you want to google that, but it has a lot of error checking, and layers of safety and security information that you may not need if you are just trying to get stared.
Hope this helps.
Mike