I only have a GT-8 but channel mode dual mono and turn both preamps on. The preamp lights won't both be on at the same time, only the one you are editing/setting but they will both contribute to the tone.

To clarify. On page 98 of the manual they show all the different channel modes but they don't explain how to change them. They are pre configured on the factory presets. Some are A. Some are B. Some are A and B. I want to switch some of them to Dual Mono. They don't explain how to do that.

Yes... I owned a few GT-8's and that's how they worked. On the GT-10 they don't explain how to configure a patch to be in dual mono mode. For instance... I pick a patch to edit. It is configured PREAMP A only. If I press PREAMP B it will turn off A and turn on B. What if I want both on and sent out dual mono mode? It was easy to do it with the GT-8. I can't figure it out on the GT-10.