Maybe I can be of help. First is it possible you have what is called pigeon breast? (have no idea how that name came about…doesn’t’ look like any pigeon I ever saw LOL. One side of the chest is just plain larger. Sometimes if the person is heavy he or she may never notice it until they lose weight. Any building over the chest could make it just that much more pronounced. That being the case not a whole hell of a lot you can do about it.
But, YES YOU CAN build one pec larger than the other.
Do SINGLE cable cross/over. In fact, I’m convinced it does a better job than doubles anyway.
Doubles allow you to use your back and body weight.
Singles allow you to use MORE WEIGHT, and nothing else can kick in unless you’re doing it wrong.
Stand feet apart (outside shoulders)
Drop body into Low Horse (semi -squat)
Keep back perfectly straight
Bend arm just enough to take stress off the elbow
Place other hand over pec being trained (concentration)
Pull STRAIGHT across chest. Do not cut downward.
And just gage what weight you want for either side.