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Generally, if there are two CAN networks, they are separate for a reason, so you can't just join them together.
There are several reasons why two CAN networks can't be joined.
1) They run at different baud rates. If the networks were joined, nodes would see messages of the wrong speeds, which they would interpret as an error.
2) Some or all message identifiers (IDs) on one network mean something different on the other network.
3) There is too much traffic on each network, so that joining them would overload the combined network.
Also, each CAN network is terminated with 120 Ohm resistors at each end, and each node is designed to drive 60 Ohms. If you combine two networks, each node would be driving 30 Ohms and might not work. However, it probably would work in most conditions, and it is relatively easy to fix.
Can you tell us what you are trying to do by joining the networks?
Theoretically its possible, with a cpu and 3 can interfaces, 2 for the 2 networks you have, and the third for the output network, the cpu would decide what data to send and when, it would also be possible to translate bitrates and 11/17 bit types.
Vehicles have at least 2 buses, one can A at 500kbps, and can B at 50 kbps, fully combining them would require indepth knowledge of the system, and even then there may not be enough bandwidth.