In my opinion, it really depends on your recipe and what flavor profile you're attempting to impart. Times I would consider pitching Brett after Sacc would be to obtain subtle Brett characteristics instead of strong ones.
For example, if you ferment your beer down very far (1.010 or lower, for example) with just the Sacc, then there will be less for the Brett to work on. However, if you design your recipe to finish primary Sacc ferment at 1.020 (via mash temp or less attenuative yeast), there's obviously much more for the Brett to work with once introduced, and you'll get more "Brett character".
There are a vast number of commercial breweries who do clean ferments and only add Brett/bugs after transferring to a secondary vessel for aging. For my Flanders Red that I routinely do, I pitch Sacc and Brett together, up front, because I've found it gets more sour that way. Conversely, for the LaFolie clone I did, I mimicked what New Belgium did and clean fermented with a lager Sacc first, then added Brett/bugs prior to long-term storage.