...why don't tour pros play with two putters - one for long putts (lag putting) and one for short (accuracy)?
I don't know. Maybe a mallet style, centre shafted for accuracy, blade style for lag? Different shaft lengths, weighting, etc.
Two of any club is a waste and Mickleson only got away with it by dropping a long iron but still had the repetoire to produce the necessary shots. If it was such a viable idea it would have been done ages ago. In fact it says more about how little faith Mickleson had in his driving than any attempt to gain an advantage on the field