I switch back and forth between just combo, just thiele, combo+thiele. I leave the amp on, and always leave one output jack occupied. Does that make sense?
Using both -> both 4 ohm jacks occupied. Switching to just the thiele, remove thiele cable, plug into 8 ohm output, unplug combo speaker cable. Or vice versa. The amp always has a load, and a number of us have proven that (with the boogies anyway) you can underload them or overload them without too much difficulty. (Boogiebabies runs three 8ohm speakers, one out of each speaker jack).