Ability question

So I have a blue shiny yanma I tend to use as a sweeper for endless runs, isn't awful could be better. It's stats are almost perfect, just missing like 2 spA points. And I have is passive sheer force. Just a bit ago I, found a pokemon with serene grace as it's main ability. If I chose to fuse them, what would happen? Would serene grace be negated cause of sheer force? Would sheer force get an extra damage boost? Or what would be probably the best option is having sheer force boosts as well as still getting moves secondary effects. Does anyone know how this would work? Or should I try it and find out later?