CS == Chip Select. Each chip has to have one output pin from the Arduino. You can use pin 10 for one chip and any other unused pin for the second chip.
The connections to MISO, MOSI, and SCK are shared between all SPI Chips. You can have multiple SPI chips connected to the Arduino and the CS pin selects which chip is being used.
My sketch is written for a single chip with 2 channels. It has channel 0 and channel 1. If you are using 2 single channel chips you will have to modify the code some for you purpose.
Here is the Define for the first chip.
#define MCP_DIGITAL_POT_SLAVE_SELECT_PIN 10 //arduino <-> Chip Select
To add a second chip you would #define a second output pin. I named it with a 2 at the end and chose pin 9 for output. You could have a different pin for chip 2.
#define MCP_DIGITAL_POT_SLAVE_SELECT_PIN2 9 //arduino <-> Chip Select
You will probably need other code changes but, that is all I can do for you at the moment.