As the sibling comment explains, it does make the other flat plate convex, but by a smaller amount. Their shapes sort of average out.

I'll add that this depends on the materials having similar properties, so when ground together, they are grinding each other. This means whatever residual shape you have in one of them, it won't be transferred completely to the other.

If the materials had very different hardnesses, one of them would dominate over the other when they're ground together.