Hey there.
What's the use case? What are you trying to achieve.
You may be able to get away with this using a route slug like:
Route::get('posts/{slug?}/{?edit}', function($slug, $edit) {});
But i'm pretty sure if you omit the first parameter, which i'm assuming you might sometimes do if it's optional, then that would break the URL, and stop it resolving.
Let me know what you're trying to do, and i might be able to help you figure out another way around it!