I have two post types in the same query, how can I use CSS to select only one post type?

The post_class() function can be used in the loop to automatically output some classes that you can target via CSS. Post types will have the class type-[post-type].

How that might integrate with Elementor is unfortunately something you will need to ask via Elementor’s support channels, as 3rd-party plugins and themes are off-topic here.