50g

The function AUGMENT (RS MATRICES CREAT AUGMENTED) can help you, but
if you have [A] and [B]
it gives you:
[A]
[B]
to get [AB] you need a little program to transpose the two matrices
then use AUGMENTED and transpose back.

Another way is to use COL+ ( RS MATRICES CREATE COL+)
Example: if you have two matrices A {3 2} in level 2 and B {3 4} in
level 1 then you want B to be added as a third column in A matrix, so
enter 3 and then COL+

A little program to get [AB]using COL+ is:

<<
OVER @ get a copy of matrix in level 2 @
SIZE @ get the size of that matrix @
2 GET 1 + @ get the number of columns and add 1 @
COL+ @ Add the two matrices @

>>