Cleaning up a matrix

I have the following matrix

     3     6     5     4     1     2
     3     1     5     6     4     2
     4     3     5     1     6     2
     3     1     6     4     2     5
     3     2     5     6     1     4
     5     1     2     4     6     3

which is created by $n*r$ permutations of $1:n*r$, here $r=2,n=3$.

I want some trick (there must be one) to get the following two matrices

     3     1     2
     3     1     2
     3     1     2

     6     4     5
     5     6     4
     5     4     6

Any help would be greatly appreciated.