Hi,
In the case of MCP2515 and MCP4911 there really can't be any conflicts since only one of them actually puts data out on the MISO line.

With SPI it's always the master (the PIC in this case) that initiates and "drives" the communication with the slave devices, the slave devices can not start transfering on their own. The INT\ pin on the MCP2515 - for example - is used to let the master device know that the slave needs attention. If the master is in the middle of a "converstation" with another device it's up to the master to decide if it should answer the slaves request for attention or wait (and possibly risk missing something important).

Again, with SPI there's one master (the PIC in this case) and, at least, one slave. The master provides the clock and only the slave whos CS\ line is active (low) will listen to data being put out by the master on the SDI/MOSI line and only the slave whos CS\ line is active will (or at least should) put data out on the SDO/MISO line. A SPI slave device can never "send" anything on its own since the clock to actually shift data out on the SDO/MISO line is generated by the master.

/Henrik.