The issue here is that 'adding two multi-dimensional arrays' is not well defined.
Imagine i have two documents printed on paper, and i want to 'combine them'. do i
a) place document 'A' on top of 'B'?
b) Shuffle them together like a deck of
cards, so i have A.page1 followed by B.page1 followed by A.page2...
c) Do I cut-n-past the contents of B.page1 physically onto A.page1
d) do i drop both into the paper shredder and mix up the resulting scraps?
All of these are valid ways of 'combining two paper documents' - each would be appropriate in certain situations.
The analogy is similar for what you are asking. We're not sure how exactly you would want to combine your arrays, so specific help is hard to give.