A CAN interface is legacy device, it is based on a technology that has fixed allocated I/O adresses and interrupts.
Therefor BnR devices (APC4100, etc.) are only able to use one CAN interface. (see also the according user manual).
Possible solutions:
1.) Thirdparty PCI/PCIe cards together with a B&R pc that support such PCI/PCIe slots
2.) CAN/USB adapter like from supplier peak systems (PCAN-USB: PEAK-System).