I’m going to omit discussing spot reduction because you’ve received very good advice on that and I have nothing to add.
Although different regions of the spine have different amounts of mobility, the spine bends side to side like you’re talking about (lateral flexion) in daily life. It also bends forward and back (anterior / posterior flexion) and rotates. Exercises that strengthen and/or stabilize typical spinal motions can be functional, provided the person doing them doesn’t have an injury that limits the motion.
So, I would say that generally, yes, you could do side bends with weights in both hands. If you’re not pre-disposed to injury with lateral flexion, it’s not going to hurt you. But it’s not a particularly effective exercise for stability or mobility.
Joe Ingram Jr’s idea, the farmer’s walk with a weight in one hand, for stability. I’m also a fan of side planks for lateral stability and side planks with hip lifts and lowers for mobility.
The question is kind of tricky.
A client who has no back injuries would be ‘ok’ for side bends with weights.
Side bends, however, are not the greatest exercise in the world. As Janet and Paul mentioned, the exercise isn’t very effective because you can’t spot reduce.
A better exercise would be farmers walk holding a weight (dumbbell, kettlebell) in one hand.
Clients may say ‘ how can I work my side muscles when they don’t move?”
Keep in mind that muscles cause movement and also act as stabilizers.
Hope that helps you out!