This is going to complicate your thoughts further, but after a very recent trip to India I found myself shooting at ISO 1600/3200 a lot more than I expected given that you are scarcely ever allowed tripods in interiors. Even outside in the markets I was very often at ISO400 given that the sun was very bright and shots containing both sunlit and shady subjects were ugly so I concentrated on shade. In short I think you need to be equipped to handle a very wide range of exposures and two bodies may not enable you to handle all conditions in B&W and colour.