Unfortunately it is not possible to reliably use the INT pin as a digital interrupt pin. When using only 1 mcp2515 after sending ~ 20 remote requests no more INT falls are detected. It seems like somehow the library blocks INT pin interrupts when sending messages, so the arduino doesnt recognize a fall and thus the int pin is never cleared.
I experienced a different problem with some mcp_can libraries but with similar symptoms, where the SPI transaction were not atomic, being interrupted by a PIN Interrupt (from another SPI device) and causing major dramas. The solution was to disable interrupts before and re-enable after each SPI communication done by the library.