I'd check and see if each of the pattern companies has a list of on-line & catalog retailers that sell their patterns and check there and see which ones they have in common. Although, those places might sell those particular brand of patterns but not necessarily have in stock the specific patterns you want. Herrschner's might be a good bet though.
Edited to add...
And if you find a source that carries both brands but doesn't have the pattern you need, I'd still contact to see if maybe they can get it for you.
Edited AGAIN to add...
I just checked and Elann has some of both FT and KPS patterns.