You're right, you can't get a brown flame that way. Basically, when you have two significant flame tests in the same flame, what you'll get is both colors, flashing back and forth, as you move the wire from one part of the wire to another, richer in one or the other salt. Or one is consumed faster, and the other remains behind. I'm guessing copper salts will disappear first, because calcium is more refractory (we make bricks or porcelain or cement for example, out of calcium oxides and other such elements, so this soluble salt may bake onto the wire.) Or one salt's color just washes out the other. For example, potassium has a purple flame, but every textbook I read said you never see it, its always washed out by bright yellow causes by traces of sodium. Unless you look through cobalt blue glass, to filter out the yellow. But I never saw anything through that dark blue glass.