Expanded shells don't cover all of the coin they are placed over.
A while back Johnson coins made a deep shell coin, but it's now discontinued.
When presenting your routine using an expanded shell, always have your audience view the coins from above.
If you lift the coins up to eye level, they will of course be able to see that there is another coin inside of the shell.
Using a closeup pad will also help, as the cushioned top of the pad will allow the wight of the coin to push it down very slightly into the pad, thus making it more difficult to see the edge of the coin.