With what expected as result, pray tell?
You can't have a 2-column matrix of 25-elements; you could keep the two as elements in a cell array (not particularly recommended, but possible, and the choice if there's going to be a need to identify those elements from A again later).
C=[A(:);b(:)];
returns a column vector; you can reshape() it by prime factors if wanted.
Alternatively, one could augment and reshape the first to have two columns -- but that will require A having an even number of elements to do so, either by adding (or removing) an odd number first.
C=[reshape([0 A],2,[]);b];
is one possible solution of any number in that vein.
