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.