My personal take on this (and this is subjective and dynamic) is that the "use different engineers" axiom is far more important when talking about moving from mixing to mastering than it is when moving from tracking to mixing.
In some cases (with the right engineer), in fact, it can arguably be advantageous to have the same engineer for tracking and mixing. Why? If the engineer is good and plans ahead from the get-go - i.e. if he already has in his head what he wants the final mix to sound like - he can make his tracking selections to fit that mix properly. This is especially true if there is no one else really taking the reigns with bosth fists as the producer.
OTOH, if you have seperate tracking and mixing engineers who are in agreement and in concert as to what the target mix will be, or who are working under the direction of a common producer, then such continuity should not be an issue, and the seperate mixing engineer can be advantageous.
Plus there is the fact that many engineers are just better at one function than the other. Some great baseball players are better at shortstop than they are at second base and some engineers are better at tracking than they are at mixing. And vice versa.
But no matter how you slice that, any good engineer will take the longest break that the schedule will allow between all phases; tracking, mixing and mastering. It's not only ear fatigue, but brain fatigue as well. It's like any problem that looks fresh when you come back at it the next morning, you often see something almost instantly that you would have never thought of the night before. I personally try to never combine engineering phases on the same project in the same day. If I have to because of schedule, I'll at least work it out so that there is a scheduled break between phases; a break where there is silence and rest.
But that "fatigue" idea gets a little grayer I think when you start talking about juggling multiple projects. For example, I'll have much less ear and brain fatigue after a 14 hour day if I spent the first 5 hours working on project A, the second 5 hours on project B, and the final 4 hours of project C than if I spent the entire 14 hours working on project A only. There will stii be fatigue either way, but it somes on much faster and harder if you don't break up your day between different chores. You can't *just* look at the number of hours, but also at how they're managed.
[SIZE=1][B][COLOR=DarkSlateBlue]Glen J. Stephan,
SouthSIDE Multimedia Productions[/COLOR]
[COLOR=DarkGreen]RECORDING RESOURCES AND INFO SITE:[/COLOR]