serial peripheral interface - DS1867

hi forum, i have a question according to the use of SPI. i would like to control the digital potentiometer DS1867. since this pot is different to the example in the arduino tutorial section, i have somehow a hard time to figure out what parameter need to be adjusted to make it work with the DS pot. the DS1867 contains two 256 position potentiometers whose wiper positions are set by an 8 bit value. this two 8 bit values are written to a 17 bit I/O shift register. even if only one potentiometer is used, this 17 bit value needs to be sent entierly to function as a valid message. since i never used to work with SPI and registers, i would mostly appreciate some hints, how to write a program that interfaces one of this pots. thanks for any suggestion, r.