In your example, the drill designer does not have a conflict of interest. He/she wants both corps to score equally well.
There could be a conflict of interest if a designer was to be paid incentives based on a corps performance. Let's say that the designer worked for corps A and B. Corps A and B were tied going to championship week at 6th place. The designer is to be paid $3,000 extra by corps A if they place 6th or higher. Which corps would you work with during the last week?
I agree with PupleKnights that as long as the directors are made aware of potential conflicts of interest this really shouldn't be an issue for DCI or anyone else.
Besides, let face the reality that Drum Corps can not afford to pay designers a lot of money. That being the case, I would expect designer to have to work for more than one group in order to make a living. Just a fact of life.