Sorry if I don´t understand what happen.
How many need you? 1 or 2 channels??
You are using one for loop channels less that 1 that you only will address one channel;
// go through the six channels of the digital pot:
for (int channel = 0; channel < 1; channel++) { // Here channel will execute only 1 time
Change your code to:
// go through the six channels of the digital pot:
for (int channel = 0; channel < 2; channel++) { // Now for will execute 2 times