Yes, as Murphy says, you can certainly mix conditionals. However, the example you gave doesn't really make any sense. In this scenario, both clauses should refer to past time time : "'If I had known he was in hospital, I would have bought him some fruit". You didn't know he was in hospital and you didn't buy any fruit - both refer to past time, not present. The fact that he is still in hospital at the moment is irrelevant: the statement refers to your ignorance of his hospitalisation and your failure to buy fruit, both of which 'occurred' in the past.