Let's give the matrix we're operating on [1 2] a name. Let's say
A = [1 2];
Then repmat repeats A in a two-by-two arrangement:
repmat(A,2,2)
ans =
1 2 1 2
1 2 1 2
Then reshape it into a 1-by-however-wide-it-needs-to-be matrix:
reshape(ans,1,[])
ans =
1 1 2 2 1 1 2 2
