I've always thought that a spread move will do the same amount of damage to two separate Pokemon each as it would for a single Pokemon. But I've only recently found out that a spread move will actually do less to each one when spread between two.
So by how much is the damage decreased for each one it hits? And is the damage still decreased if one of the Pokemon somehow isn't hit by the move? And has this been the case ever since gen 3, when double battles were introduced?
They will be decrease to 75% of their power. For example, The power of Earthquake will become from 100 to 75 for each target in doubles. If that move like Earthquake only hits one target in doubles, then it will be the usual full power which is 100 for Earthquake. It has been the same from Gen 3 onwards.
Credits goes to Pokemaster and some research.