Thanx peter for the code I have few comments
1. Why are you using two bytes array, with c language short int is 16 bits and long int is 32 bits, you can make your program take advantage of the int and shift the 16 bits in or out in one for loop.
2. are you using 2 8-bit shift registers or did you cascade them to make 1 16-bit shift register?
I guess it all depends on your hardware configuration