Some people don't try to fit them together--they just lay the two layers, and then cut the wavy edge in place.
I would first cover the whole tier with the white. Then the blue goes on as a thin second layer. After you cut the line through just the blue layer, peel off the excess blue fondant.
The silver overpiping hides any gaps.
That is what I was going to do. I hoped that if I made the blue fondant piece thin it would be difficult to tell. Thanks for responding!
One other thing you could do is to wrap the white layer around the cake sides. Then drape the blue layer over the top, like normal, then cut your pattern with an Xacto knife, through both layers, then slide the white part out from under the blue. I did this with a a similar design. Freeze the cake first so it doesn't get messed up when you take the excess off. Alternately, you can just leave the white there.
Or just roll out a piece of white fondant, cut the wavy shape out, and lay it on top. smooth it down, and then use an extruded rope to hide the seams.
Thank you all! I think I'll go the easy way for now and leave the white under the blue and just roll it out thin.
And thanks Annabakescakes for the freezing tip. I hadn't thought about that!
Happy Fourth of July to you all!!