I’m having the same issue when printing PETG with PLA interface supports in my X2D. Aux fans ramp up to 80, 90 or 100% once PLA is extruded for the first time. For those of you who do not want to deal with custom G-code, @rinkel found that raising the PLA softening temperature stops the aux fans from spinning up, and @FedX’s analysis shows the auto-cooling thresholds are driven by min_vitrification_temperature (the internal name for that field). So in theory all you have to do is edit the PLA filament settings (preferably create a custom one for supports use), and change the Softening temperature from the default 45C to 60C (the same value PETG uses). I also lowered the Auxiliary part cooling fan setting from the maximum of 70% to 40% (the same value PETG uses).
Will report back once I try this settings myself, need to wait for a long print to finish.