A more generic version of Stephen's method 1:
skipcolumns = 2;
bcols = (1:size(b, 2)) * (skipcolumns+1);
assert(size(b, 2) * skipcolumns == size(A, 2), 'number of columns in A and b don''t match skipcolumns');
acols = setdiff(1:size(b, 2)+size(A, 2), bcols);
new1(:, bcols) = b;
new1(:, acols) = A;
a more generic version of method 2:
skipcolumns = 2;
assert(size(b, 2) * skipcolumns == size(A, 2), 'number of columns in A and b don''t match skipcolumns');
new2 = reshape([reshape(A, size(A, 1)*skipcolumns, []); b], size(A, 1), [])
