Completing some Partial Latin Squares

We show that any partial 3 r × 3 r Latin square whose filled cells lie in two disjoint r × r sub-squares can be completed. We do this by proving the more general result that any partial 3 r by 3 r Latin square, with filled cells in the top left 2r × 2 r square, for which there is a pairing of the columns so that in each row there is a filled cell in at most one of each matched pair of columns, can be completed if and only if there is some way to fill the cells of the top left 2 r × 2 r square.