I know that $6\times 6$ MOLS cannot be constructed, but if I am not mistaken we can draw up two MOLS that are $6 \times 6$ with $34$ distinct pairs of symbols. However, I am not able to find this construction of MOLS. Could someone show me what it would be like?
[EDIT: MOLS = Mutually Orthogonal Latin Squares]
It was found by Joseph Douglas Horton, Sub-latin squares and incomplete orthogonal arrays, Journal of Combinatorial Theory, Series A, 16 (1974) 23-33. Unfortunately, I don't have access to this journal. Interlibrary loan could probably help you.
Edit (Douglas S. Stones): The JDH paper gave a pair of orthogonal partial Latin squares, which I edited to give this pair: