The concern is cross-pollination. If you aren't saving seed, I don't think it's as much of an issue.
Otherwise, when you went to grow out your seed the next season from the cross-pollinated corn, they wouldn't breed true. You'd have a bunch of different hybrids.
You might get some that are edible as sweet corn, but with different characteristics of flint, and vice versa. My concern would be not only taste in the next generation of sweet corn, but what if the flint-dominant hybrids didn't have the preferable flint characteristics you were looking for when you planted them the first season? What if storage or ripening characteristics were wildly different?
I have only ever selected single cultivars at a time, as I work usually on a small scale, but I don't think cross-pollination between different cultivars could affect yield or performance in the first year. Also, if they don't flower at the same time, the pollen of the first might be gone long before the flowers of the second would be ready.
A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.
-Robert A. Heinlein