Here comes one of Paul Vaderlind's orthogonal sudokus (orthogonal in the same meaning as for latin squares).

Fill each cell with a two-digits number from 11 to 99, without zero digit, in such a way that

a) The first digits represent one regular sudoku grid.

b) The second digits represent one regular sudoku grid as well.

c) (orthogonality) Each number from 11 to 99 (without the digit 0) occurs exactly once in the grid.

59 .. .. 18 .. .. 84 .. ..

.. .2 .. .. .. 94 .. 43 ..

1. .. 74 .. 63 .. .. .. 98

.6 9. .. .5 7. .2 6. .. .1

6. .. .2 2. .. 5. .9 .. 1.

.4 .. 1. .6 3. .3 .. 9. .2

93 .. .. .. 15 .. 31 .. 6.

.. 85 .. 91 .. .. .. .9 ..

.. .. 47 .. .. 69 .. .. 25