in your given example, either dimension should be same, A and b should have same number of columns. I think you can make your 2-D matrix or row or column array.
A = A(:);
B = B(:);
then union function will take the union of A, B. You can convert both in 2-D matrix by reshape command.
and If you have to concatenate A, B then again both A, B should have same number of columns.
