Isolated CAN transceivers as a CAN bus extender
According to your schematic, you broke up the bus in two and there's no termination anywhere. You get the wrong impedance instead of the expected 60 Ohm. Before each transceiver on the "hack" device, you need terminating resistors of 120 Ohm between CANH and CANL. 

In addition, the "hacker" MCU's CAN controller must actively participate on the bus, you can't have it set to listen-only or loopback mode. Otherwise the other node on the bus will have none to speak with and eventually go error passive mode.