The 2 units are connected structurally, there are only 2 ways to reduce this problem.
1) decouple the structure/ essentially breaking the connections between the 2 units..( very expensive and a lot of construction involved)
2) absorption of the impact noise at the source side. essentially putting in carpet underlayment, padding and carpet, better foam seals on doors...applications that reduce the force of impact before the structure is involved.
Trying to soundproof your side would be like chasing your tail..you would be just moving the sound from one area to another....to be effective you would have to soundproof all 6 sides of the cube.
Probably not the news you wanted to hear but you need to understand that vibration is traveling through the structure at 13,000 fps and remits at any area of least resistance.
If you want to reduce human conversation or TV noise we could do that with cost effective solutions.
Impact not so easy.
Let me know if you want to reduce the human speech aspect of the problem and I would be more than happy to give you options.
Randy S.