so moving b in to a won’t work if some subfolders are already there

I though that if they are already there you would merge / overwrite, isn't that what you need ?
a/b/c/2.txt seems like the result of moving b from dataset B into folder b in Dataset A, what am I missing?
(My assumption is that you have both datasets locally on the same machine and that you can just copy the files from b of Datasset B into b folder of Dataset A)