Strange question. You can already do that, you just chose not to, by shifting out digits[i] and using i twice. If you used some value other than 'i', that's what would be displayed... example:
shiftOut(dataPin,clockPin,MSBFIRST,digits[i]);
shiftOut(dataPin,clockPin,MSBFIRST,digits[k]);
If your question is, "how do I choose a different value of 'k'?", then you need to just study C. There are so many tutorial sites online, all you need to do is Google them.
For demonstration purposes you could do something like
k = (i + 5) % 10;