Hi,
The idea with SPI is that you can have multiple devices on the bus and then select which of them you want to communicate with by pulling that specific device chip select line low (it's always active low as far as I know).

SPI and I2C are not compatible with each other and SPI devices and can not share bus with I2C devices. If you're using the MSSP module in the PIC (which can do SPI or I2C but not both at the same time) for the SPI chip then you're left with the software I2C commands - which can use "any" pins you like. Some PICs have two MSSP modules so you could set one up for SPI and the other for I2C.

With that said, the datasheet for the MCP4911 seems pretty clear that it IS using SPI and not I2C, where did you get that?

/Henrik.