Hello. If I do my MSc without the dissertation at a university A, can I join a university B to only do the dissertation part of a similar MSc and earn the MSc from the university B ? How much (in percentage) will the fees reduction be, if any?

I've never heard of anyone doing this. Do you mean do the research at one uni and then submit it at another? Or do a taught masters and one uni and the research component at another? The first option, I would say probably not. The second option might be a possibility but highly unlikely.

I expect you would have to pay fees twice if it was even possible.

Why do you want to do it?