Multiplexing a CAN bus
In principle you should be able to multiplex MCU CAN signal lines, prior to their being used to interface to a CAN physical layer transceiver, using something like a 74HC4052.

In practice, I'd suggest that it's not a very good idea to do so.  The problem is that multiplexing would mean that you are guaranteed to periodically miss transmissions from each of the buses.

If you can't tolerate regularly missing traffic on either/both buses, then you'd be better off using an external CAN peripheral; an example would be the MCP25625, which has a CAN link layer and PHY and is controlled via SPI.  Adding this to your existing MCU + CAN PHY would allow you to simultaneously and separately interact with both CAN buses, with little if any risk of missing any traffic on either.