User blog comment:Mamerge/Do they stack that way or this way?/@comment-104.157.27.122-20140823175456

Most +50% or the like leader skill effects are  cumulative , so in your example 25% + 25% = 50% ATK boost.

This also spills over to item effects, wherin the same example would result in a 100% boost to one aly if you used a Brute Elixer on it (25% + 25% + 50% = 100%).