In both instances--starter or harvesting--you'll change the ratio of sacc to brett, because they grow at different rates.
In the case of the first pitch, this may not be a problem. Saison yeast does pretty well under-pitched, and the brett will work fine over time with a small colony.
In the case of harvesting, this may not really matter. Depending on when you harvest (how many months after pitching), you will get a healthy colony of both yeasts. You may get a different character in your next beer, but I would think it would be in the same ballpark.
(N.B.--I actually haven't used this yeast, although I will be pitching it into a beer next week. I'm going off knowledge of how these two species act in general.)