When I bought my bike, I found out that the previous owner did that. But even after that both plugs were in terrible condition. Burned black with wrong gaps and never maintained. I adjusted my valves, cleaned and adjusted my carb, and actually switched back to CR7E. To be honest, I have no problem since that and I see no reason not to follow the original service manual. In my opinion if everything is OK there should not be a big difference what spark plug is being used in a second cylinder. Yes, it's a little bit hotter, but both plugs (CR7E and CR6E) should have no problem with those second cylinder temperatures.
What to say, just test Yourself, what is better for You. You can not burn Your engine anyway: )