Conditional logic to decide which page to display?

Hm, sounds a little out of scope for this plugin, but you can totally make that work with a little bit of custom javascript in your theme.

You page would include the shortcode for both pages. Example:

[insert page='your-first-page' display='all' class='custom-class-for-first-page']
[insert page='your-second-page' display='all' class='custom-class-for-second-page']

And the javascript would look something like the following, assuming you’ve defined a custom function called getParameterByName() to get your querystring value (let’s assume it’s show_page=first or show_page=second):
https://stackoverflow.com/questions/901115/how-can-i-get-query-string-values-in-javascript

if ( getParameterByName( 'show_page' ) === 'first' ) {
    $( '.custom-class-for-first-page ).show();
    $( '.custom-class-for-second-page ).hide();
} else if ( getParameterByName( 'show_page' ) === 'second' ) {
    $( '.custom-class-for-first-page ).hide();
    $( '.custom-class-for-second-page ).show();
}