When stacking various modifiers like shell dispersion or extra credit gains, they always add to each other, rather than multiply.
Examples
Bonuses
If you have multiple bonuses credit earnings from boosters and premium time, and the values are 20% and 50%, the sum total of additional credits earned per battle would be 70%.
Combat Bonuses
If a Commander skill like Velocious increases your ship’s movements speed by 2% and you have a Common Booster of Battle Propensity with 2% extra of the same stat, your overall movement speed increase will total at 4%.