If I have 2 Double Turbo Energy Cards attached to one of my Pokemon, would its Attack damage be reduced by 20 or 40?
It would be reduced by 40 since each card's reduction would apply.
Source: Brilliant Stars FAQ; TPCi Rules Team (2022-02-24)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH9_151
SWSH9_151