Hey Lars,
If you are planning on using dual tones with the POD HD500, it is very normal for you to have to listen to both amps at the same time while adjusting your tone. The whole point of using dual tones is to layer sounds, if you adjust them in while they are isolated, it will be inevitable that you will have to do additional adjustments after you combine the two together cause of the way their sound changes when you combine them.
-Don
Customer Support
Line 6
